#10-32 takes a #21 drill bit, .159". A 5/32 is a few thousandths smaller but by the time you get it drilled it will be nearly the same. You can buy proper tap drills packaged with a tap at most any decent hardware store for a few bucks just to get the drill you need in a pinch. Most lock bolts I see are #8-32, make sure you know what you have.
I assume you have a trigger plate if it's a fusil. You locate the trigger based on the lock's sear arm FIRST, everything about the tang bolt angle and location is based on that, so you slot the stock and hang the trigger in the right place (there's a lot to know about that, don't just jump into it if you don't know all the location, angle, distance, etc.). Once you get the trigger hung and working the lock properly, then you inlet the trigger plate to suit the trigger position. Then you locate where your tang bolt needs to enter the trigger plate in front of the trigger, draw a line up to the tang (I like a slight angle back to the bolt so the bolt head is "square" to the peak of the curve in the tang), draw a line between those two points, drill UP from the bottom along that line halfway through the stock with a tap drill the size for the tang screw threads, remove the barrel and drill/countersink a hole the tang bolt will pass through, then reinstall the barrel and drill with the tap drill down at the angle to meet the one you drilled up from the bottom. Carefully drill all the way through from the bottom to smooth out the hole. Install the trigger plate in the inlet and drill the tap-size hole through it from the top, through the tang hole. Then run your tap through the tang hole and cut the threadss through the trigger plate. Taps are very brittle and easy to break, especially cheap 4-flute ones, you want some heavy oil or grease on the tap and in the drilled hole in the trigger plate, go a quarter to half turn at a time, back it off to "break the chip", go another quarter to half, repeat repeat repeat and clean the chips often.
Lock bolts are basically through the stock level and square from the bolster to the side plate. You have to study where the top side plate hole needs to be based on its design and try to make that hole line up in a good spot on the lock bolster AND either miss the breech plug or go through it solidly, not catch the edge or you can't drill a straight hole. There's a workaround to clear the breech plug if you have no choice, you can notch it with a file to line up with your holes after you drill them through the stock. You might have to fudge a bit but drill a smaller than tap size hole in the bolster first once you get it worked out, then drill halfway through the stock from the lock side, repeat on the side plate side. Once you get a hole through the lock, stock, and side plate, change to your tap drill and drill the lock plate through FROM the side plate side, then remove the lock and drill through with a clearance size bit for your bolt. then reinstall the lock plate and tap the hole from the side plate side. This keeps the threads in-line with the hole which is critical to not cross threading the lock bolt later. For the front lock bolt (if it has one), hopefully you got your ducks in a row when you laid out the stock, barrel, and lock positions because the bolt has to go between the rammer hole and the bottom of the barrel and come out in a good place on both the lock and the side plate. This bolt is drilled just like the main lock bolt, mark your lock and side plate and drill them, drill halfway between with an undersized bit, make sure you don't hit the barrel or rammer channel, run the drill all the way through, step up to lock bolt tap size, drill through from the side plate side, remove lock, drill clearance size through the side plate and stock, reinstall lock, tap hole, done.
Clear as mud? Do we need to visit how to set up the trigger location first?
I assume you have a trigger plate if it's a fusil. You locate the trigger based on the lock's sear arm FIRST, everything about the tang bolt angle and location is based on that, so you slot the stock and hang the trigger in the right place (there's a lot to know about that, don't just jump into it if you don't know all the location, angle, distance, etc.). Once you get the trigger hung and working the lock properly, then you inlet the trigger plate to suit the trigger position. Then you locate where your tang bolt needs to enter the trigger plate in front of the trigger, draw a line up to the tang (I like a slight angle back to the bolt so the bolt head is "square" to the peak of the curve in the tang), draw a line between those two points, drill UP from the bottom along that line halfway through the stock with a tap drill the size for the tang screw threads, remove the barrel and drill/countersink a hole the tang bolt will pass through, then reinstall the barrel and drill with the tap drill down at the angle to meet the one you drilled up from the bottom. Carefully drill all the way through from the bottom to smooth out the hole. Install the trigger plate in the inlet and drill the tap-size hole through it from the top, through the tang hole. Then run your tap through the tang hole and cut the threadss through the trigger plate. Taps are very brittle and easy to break, especially cheap 4-flute ones, you want some heavy oil or grease on the tap and in the drilled hole in the trigger plate, go a quarter to half turn at a time, back it off to "break the chip", go another quarter to half, repeat repeat repeat and clean the chips often.
Lock bolts are basically through the stock level and square from the bolster to the side plate. You have to study where the top side plate hole needs to be based on its design and try to make that hole line up in a good spot on the lock bolster AND either miss the breech plug or go through it solidly, not catch the edge or you can't drill a straight hole. There's a workaround to clear the breech plug if you have no choice, you can notch it with a file to line up with your holes after you drill them through the stock. You might have to fudge a bit but drill a smaller than tap size hole in the bolster first once you get it worked out, then drill halfway through the stock from the lock side, repeat on the side plate side. Once you get a hole through the lock, stock, and side plate, change to your tap drill and drill the lock plate through FROM the side plate side, then remove the lock and drill through with a clearance size bit for your bolt. then reinstall the lock plate and tap the hole from the side plate side. This keeps the threads in-line with the hole which is critical to not cross threading the lock bolt later. For the front lock bolt (if it has one), hopefully you got your ducks in a row when you laid out the stock, barrel, and lock positions because the bolt has to go between the rammer hole and the bottom of the barrel and come out in a good place on both the lock and the side plate. This bolt is drilled just like the main lock bolt, mark your lock and side plate and drill them, drill halfway between with an undersized bit, make sure you don't hit the barrel or rammer channel, run the drill all the way through, step up to lock bolt tap size, drill through from the side plate side, remove lock, drill clearance size through the side plate and stock, reinstall lock, tap hole, done.
Clear as mud? Do we need to visit how to set up the trigger location first?
