It's hard to tell from the picture but your tang does appear a bit thick.
Please don't do this backwards, even though you are at a disadvantage with the pre-inlet. Inlet the barrel and tang FIRST, keeping in mind the height of the pan and vertical center of your barrel. Your lock inlet is already positioned so the barrel must be moved to align the flash hole with the pan vertically and fore/aft. Somehow you have to bend your tang and make sure it fits the stock. If the tang is proud it can be filed down some at the top to meet the wood, but not too much, so hopefully there is enough wood there to sink the tang when the flash hole is vertically aligned with the pan properly.
Do the barrel first, THEN finish sinking the lock to meet the barrel. The lock bolster must fit tight and flush to the side barrel flat and at the same time the perimeter of the lock plate inlet must support the lock plate so it doesn't tip or angle away from the barrel flat when the screws are tightened. The plane of the side barrel flat must match the plane of the lock plate inlet, even though they are at different depths. You cannot properly inlet the plate unless the barrel is in its FINAL position in the stock. Make sense? If you inlet the bottom or one end of the lock plate too deeply it will not be supported properly when tightened down and you'll get gaps between the lock bolster and barrel that collect fouling and let junk down into the lock mortice. I know it's in shadow, but look closely at the LH Haines gun above and note how the lock bolster makes an air-tight seal against the barrel flat. This face-to-face fit coincides with the perimeter of the lock plate inlet in the wood so it plunks down flat and fully supported against the barrel and stock simultaneously and tightening the lock screws only pull it down tight without tipping or flexing the lock plate.