OP, I have owned three Superformance Cobras. The first was a 351W with 400 hp, the second with a 351 bored and stroked to a 418, 550 HP. My current SPF has a 496 FE motor with 666 lb-ft torque.
If you have not owned a Cobra before, and you want to drive your car comfortably, a 400+ hp 351W based motor is an easier car to drive than a BB with big torque numbers.
Most of the dealers will direct you to a 351W based motor for the same reason, it is easier to drive without as much chance it will get away from you.
I am not trying to tell you what to buy, only what I know.
LOL at the ERA owners thinking their car is 'more original". Who cares? (besides you?)
Good luck on your build OP!
E