X-ray is an interesting idea.
I'm not sure whether it will clearly show flaws in the solder join though.

Also, with the use of tin/lead solder at the mono to barrel join, X rays bring further problems.
If the X ray did show flaws in the join the best time to discover this would be before any more work is done to the barrels like rib laying etc.
Any flaw with the join is not going to be fixed merely by reheating and re soldering.
The barrels will both have to be removed from the monoblock, be cleaned and prepared again and refluxed and retinned and reassembled and resoldered.
It would be far easier to do this before the ribs are laid of course.

So let's assume that you get a good showing of the join in the x ray and all appears well.
This may happen on the first, second, third or tenth time, it really doesn't matter, you now have a good tin/lead join.

Time to go ahead then and finish the barrel set, and lay the ribs along with any other attachments, barrel hook, sling eye, sights etc.

Now for the bad news.
You have a good or perfect join at the barrels to monoblock.
You know this because of the x ray, but that x ray is about to become worthless because when you lay the ribs etc the temp required will remelt the monoblock to barrel tin/lead join.
The perfect x rayed soft solder joint has now been compromised by being remelted after the x ray was done.

Any clearance between the barrels and monoblock will worsen the problem by allowing movement to occurr at the join.

Another x ray will be required and if it were to show that the join is no longer perfect the entire barrel set will have to be stripped and redone with the possability of the same thing happening all over again.

A quality join of barrel to monoblock using a silver or high temp solder can eliminate many of the problems regarding the use of tin/lead solder for this join, IMO.