I believe that the blacktop main caps are a weak point - not a problem with the silver top engines.

To be honest, it doesn't sound like you need to bother with a black top, for the hassle it could be why not just stick with the more common silvertop approach? at least you know where you're up to. It's not like you won't get the power out of the silvertop, how many have you seen well in excess of 300 horse?

I certainly wouldn't bother with the blacktop, why complicate things? Sure it has small advantages, in some areas like the rod/piston ratio - but you'll probably swap them for aftermarket ones anyway, and if you didn't you can swap the pistons and rods into a silvertop block.