I can't tell if that is a straight barrel or not.
If it is a straight barrel (instead of a swamped barrel) you really do not have to remove the breech plug to bed the barrel.
By now, you should have removed enough wood from the sides of the barrel channel so the barrel can be installed and removed rather easily.
If the barrel is a straight octagon, you can install it backwards with the muzzle towards the breech to find the areas of wood that need to be removed to get the barrel down to the bottom of the pre cut channel and tight against the back of the channel where the rear of the barrel will seat.
Do as the others mentioned to determine exactly where the face of the installed breech plug is.
Then, using a black felt tip marker blacken the side of the barrel and transfer the distance from the muzzle to the breech plug to the outside of the barrel. Make a nice scratch in the felt tip ink.
Compare this scratch with the center of the locks pan. If it is not at least 3/16" behind the center of the pan you will have to move the barrel back towards the butt of the stock until it is. The only way to do this is with a chisel and a LOT of TIME.
Before worrying about the tang, the rear of the barrel MUST BE TIGHT against the wood at the rear of the barrel. Gaps, like I see in your picture will not do.
If the barrel is swamped you will have to remove the breech plug as the others have told you to do.
The marking of the breech face, the fitting of the sides of the barrel in the barrel channel, making sure the bottom of the barrel is resting on the bottom flat of the barrel channel are the same as a straight octagon barrel.
Have fun and GO SLOW! A little screw up will take more time to fix than the little extra time it takes to do it right.