as rich stated, taller mains would yeild more flex. not to mention, they would require longer studs which would stretch more. to compensate for that, theyd have to be drilled out and use a larger diameter fastener for more clamping force.
i like the idea of an integrated main girdle and caps. itd basically be like bolting a whole other set of main saddles to the block. a nice experiment would be to take a complete 7M block, cutting the deck off it so youre left with just the main saddles. the only catch is that both blocks have steps cut into them to locate the main caps. hmm, maybe it would need some inserts or square spacers to lay down in the block. a poor man's main girdle. plus, youd know that using a 2nd block skirt as a girdle/main cap set, itd already be clearanced for the crank throws, lol.