All depends on how much you are willing to spend, how much time on the street if street use is planned, etc. Aluminum blocks are expensive, so It's a big choice to make. Also going to take special attention to bearing clearances, warm up proceedures and such. No reason you can't run one, but they aren't necessary unless weight is your primary concern and expense is not. Not needed at 1000 hp, and you'll likely have better engine longevity with an iron block. Max effort engine where cost is not a concern, every pound counts, and you'll be checking the engine on a regular basis, aluminum. Don't confuse the aluminum block factory engines in cars like the Corvette either, those engines are designed to run for thousands of miles under normal street driving conditions. A 1000+ hp motor is a different animal altogether.