Using screws on the rib required high precision drilling tapping and chamfering. The only way I would attempt it is to super glue the rib in place then drill and tap using a milling machine. It is far more work than soldering. Also there is a gap between the rib and the barrel that invites rust.
Screwing the thimbles to the rib is just an ugly hot mess. The ramrod should ride on a smooth surface all the way down the rib, that includes the thimbles. . TC just slapped the thimbles on top of the rib and screwed them in place. IT is the ugliest possible way to attach the thimbles. IT also invites disaster in a misplaced tapped hole, if you try to do it that way. IF one thimble is slightly off the ramrod will not fit smoothly. I'm pretty sure TC used 5-40 NS screws. You will have to buy a tap and make a chamfer tool. Todo a good job requires a milling machine for precision.
TC did it that way as a cost savings measure on a production basis. It is not a good way to do one at a time in a small shop.
It is just so much easier and better to solder the rib and new thimbles as traditionally done. File notches in the rib for the thimbles. Flat off the thimbles. Tin the rib, barrel, and thimbles. Use 3/8" all thread to align the thimbles. Wire and wedge it all. Heat, add solder, done. It will be perfect with no special measurements. It will ring like a fine double barrel shotgun.