If you were racing at Bristol, then I would probably opt for the 2.750" mains.
But for 99% of the Cobra applications out there, a Windsor block is more than adequate....even up to 7000 rpm and higher. There are plenty of 393 and 408 Windsors that will spin to the moon.
400hp out of a 351W is basically just a good intake, cam, and carb swap. You don't need to rev the thing over 5000-5500 to get that kind of power.
A 351W EFI engine is a good choice as it's already set up for a roller cam. Throw a bigger stick in it, a carbed intake and carb, non-TFI distributor, freshen the gaskets and rear main seal up, and call it a day.