If you don’t really care about the “correctness” of the engine I would suggest an LS engine. Most factory SB 400 blocks are two bolt mains and even when converted to 4 bolt splayed caps are still weak. If the plan is to purchase a Dart or World Products block you might as well purchase the GM LSX block. It has provisions for stock old school SBC engine mounts and will bolt up to any GM trans. The price of good parts are on par with std SB stuff. You will spend a bit more on a good set of cylinder heads (AFR 225cc) but a correctly built 400 cu.in LS engine will run a train on a std 23° SB and even a 18° SB for less money. The only pricey part would be the Hawk stainless LS headers for the thirdgen, but ask Oroc they are top notch. You can either choose EFI or stick with a carb setup. Either way you can’t lose and you will have a reliable 600hp machine