You will need different mounts for a big block. Also you'll need stiffer springs to carry the extra weight. I would also recommend replacing all the old suspension components when doing the swap. Going with at least front discs would also be a good idea, you'll need them to stop after nailing the gas for the first time. What big block are you contemplating? FE or 385 series? Transmission will need to be changed too if it's an automatic, if it's a manual 3 or 4 speed Toploader, then you'll need to get a big block bellhousing.