from looking at the older blocks vs the roller blocks, the differences aren't huge, but if you look at the main webbing, it's centered more in the saddles than the roller blocks-their "meat" is shifted off to one side. Also, the older blocks have shorter lifter bores which, if you want to run a roller cam, will necessitate the need to either run the bar style lifters, or running a small base circle cam with the stock roller lifter setup (which also requires drilling/tapping holes for the spider)...
another thing you might want to look at with the blocks is the deck height, from '73-'76 the deck height was 0.020" higher (to lower the compression in the smog era engines). Of course you can have it machined off, but just keep it in mind if you choose to use this block since your compression will most definitely be altered from what it's supposed to be.
some of the older blocks are also rumored to be cast from a higher nickel content alloy, but i can't tell you for sure...
all that aside, I'm not positive that the blocks will handle any more or any less power than a standard roller block-aside from getting a mexican block, or mexican caps and mating them to your old style block.
