I use Hi-Force 44 from Brownells for joining barrels to the monoblock and also to solder the spacers and the ribs. It makes the whole barrel assembly much stronger. It is 4% silver and 96% Tin and it melts at 475 degrees F. It will give you 28,000 pounds of tensile strength when properly fitted and applied. That's all that is needed except for clean joint sweating. Good Luck.
Bob