This is a solid hooked breech system, if the two pieces are mated properly, and they appear to be in your photos.
The barrel and the tang are inlet into the barrel channel and the stock respectively and cannot move laterally or down either. So the barrel cannot move forward until it is lifted by the muzzle...the barrel slides/keys/wedges prevent that until they are removed.
Noting the obvious: the easiest way to inlet that long tang is to solder the tang to the breech with soft solder so that the barrel and breech are one unit once the barrel is fully inlet. Once joined, you can sight down the barrel to ensure that the tang is directly in line with the barrel's centre line - many are not as they come out of the box. Some tweeking between pins in a machinist's vise may be necessary...bent cold. In addition, that is the time to ensure that the profile of the tang aligns with the drawing/blue print you have of the rifle you intend to create. The tang also, at this time, needs to be filed down to the barrel...the tang casting is larger by some measure than the breech and needs to be dressed down.
Be sure to maintain centre lines on your stock wood throughout the build.