Pete's idea is a good one, however; I have one on a sb1 frame and have loaded up to the max load with aa1680 with no problems. Tim and I deduced that the topper came fitted with a 30/30 barrel, the sb1 is a stronger frame than the old topper, and the case head on the 357maxi is smaller than the 30/30, so all that translates into less (or at least the same) breach thrust than the 30/30 on a stronger frame.
(Disclaimer, I think this is all correct......typing from memory and you know how that is.)