Fred,
No matter if you use heat stop, you should wire the ribs and forend lug in place just like you expect the solder to melt around the ribs. Then there will be no surprises for sure.

The good thing about Brownells is that they provide the tensile strength for most of their solders. If I'm not mistaken, the solder's melting temp follows the tensile strength. That is; the higher the melting temp, the higher the strength.