Random tips: I solder a piece of brass to the top of the barrel near the breech. The piece overhangs the sides of the barrel. Stick a dowel tight in the muzzle. Now when I lift the barrel and tang out, I lift it straight up. This avoids "levering" the tang in and out which always transfers inletting black to the rear of the tang inlet, even when it is not touching.
Get the tang bent close to the right profile. Generally I do this by peening it while it is held over a thick piece of round stock. I don't have so much luck with the adjustable wrench approach.
File some (but not enormous) draft on the sides of the tang and have some draft on the lug. Then draw-file the sides of the tang and get them nice and smooth with a fine file.
Square up the breech inlet if you have not done so. Get a good strong smooth shoulder for the barrel to back against.
Make sure you have the top of the stock profiled the way you want it, with maybe 1/16" to spare. Make sure there is a clean smooth flat extending down the wrist in the plane of the top barrel flat, in the place where the tang will be inlet. Now put a piece of wood that is tang thickness in the bottom flat of the barrel channel at the muzzle. Then use your inletting black judiciously or candle soot the tang. Set the barrel in place, straight down, so it rests on the tang at the breech and the barrel is sitting on that piece of wood in the barrel channel inlet. Now it is parallel to the barrel inlet.
Adjust the angle of the tang one more time if you need to. Now go ahead and clamp the barrel down at the breech and scribe the tang inlet. Unclamp, lift the barrel straight up and out and set it aside where it will not fall down. Been there.
Use extremely sharp, thin angled chisels for the vertical cuts at the sides of the tang and don't try to get more than 1/16" deep per "stab". If you try to get more, or use a thick-angled chisel, you will compress (and move) the wood from your intended line.
To prevent cutting too deeply or losing control when it's time to chisel out the wood, I first make some vertical stabs every half inch or so at right angles to the bore in the designated inlet. Now when cutting out the wood with a flat chisel and mallet, I hit these "stops" and have good control. Grain is often difficult here.
Now it's just a matter of removing the wood, re-blacking the tang, thinning the shim at the muzzle, and repeating this process till it is in and the shim at the muzzle is gone.
Many other ways, I am sure.
