I like the weight of a Dart Aluminum block engine as a base for a good build-up for a Cobra. >500 Hp and >500 Ft-lbs on a pump gas motor that weighs less than a 302 iron block engine. Getting 427 displacement is easy with this block.
Check out one offered aluminum block crate engine at this link from RDI that uses the FRPP aluminum block not the Dart block. They claim 535 Hp and 534 ft-lbs of torque with a 470# engine with 9.7:1 compression. Heck that more torque than my 7.4 ltr Ford deisel PSD engine and that engine weighs >1000#.
RDI Engine No. 6 Aluminum Engine Package
One thing though - bring lots of $$$