IMO, a hooked breech gun like yours should not act like it has a lot of binding when the barrel is down in the stock so the key can be pushed thru. The Key should be a snug fit but that's about all.
Because the installed key determines where the barrel is in relation to the stock, adding glass bedding compound won't accomplish a thing if the breech is binding before the barrel is down to where the key can be installed.
It is possible the breech tang wasn't inletted quite perpendicular to the axis of the installed bore.
Before worrying about it too much, what kind of accuracy are you getting? IMO if the gun is shooting to the same point on the target, then it's telling you there isn't a problem.
If it is hitting all over the place with every different load you try, then be concerned.
Rather than trying to reinstall the tang in the wood, you might try filing a VERY little amount of material off of the lower area of the barrels breech plug, or off of the forward side of the hook on the barrels breech plug.
Either way, it will allow the barrel to "rock" down into the stock further.
If you try either of these methods, go REAL slow. File a little, try it to see what it did, file it a little more...
It doesn't take much material off of either location to make a lot of difference in the fit.
When your done, IMO, the barrel should drop into the stock almost enough for the key to be installed.
With just a "little" pressure on the top of the barrel, the key should slide thru.