I would not do it the way you are contemplating. I would obtain a base high enough to fit and screw both blocks to the barrel. Perhaps even make the base yourself if you have access to a milling machine. If you wish to sweat a mount block to the barrel, soft solder will be quite strong enough..especially with the recoil one might expect from this piece. If resin flux is used as opposed to acid flux, and care is taken, there should be no need to reblue. Contour the base to fit, taking care to level the sides of the block 90 degrees from the level of the top flat of the barrel. The top is irrelevant as it will be left high to be dressed down and level after the solder job.. Mark on the barrel the edges of the base and remove all blueing inside this area. Soldering talc applied around this area is a really good insurance policy. Tin the area which was cleaned and fluxed. Apply the sub base and tension it. A wooden block will insulate the sub base from the spring tensioner(I would use the same arrangement as when soldering on a front sight ramp. Apply heat to the barrel and the sub base trying to bring the sub base to temp a bit sooner. When solder is seen at the edges the bond is done. A quick wipe with a cloth will remove excess solder, saving time later. Don't overheat as the strength will be compromised, and don't cool too quickly as this too will damage the strength of the solder joint. I prefer 'Force44' from Brownells, low temp and quite sufficient joint strength.. A single screw could esily be applied to totally insure the bond never sperates.
