Jose,
I would think silver solder would be a better choice. You would need to be careful, as the barrel tubes are held into the monobloc with silver solder, but, if worse came to worse, you would take it all apart, clean the solder joints in the tubes and resolder the whole thing back together. The gun could probably stand a rib relay anyway, at this junction in time.

I would be hesitant to use glue to hold the sub tubes into the bores of the old barrels.

Good luck.

Best,
Ted