Gears

If it were a drag/street car, I would suggest 3.91 or 4.10.
For what you say you want to do?.... 3.55s . For a 400hp motor, I think a good compromise gear would be 3.23, and they could work for you, as could 3.73. But i think you'd be happy with 3.55.
(This coming from a guy with 3.91s behind a 500hp 440, 4.57s behind a 500hp small block, 4.10s in a stock rr, and 3.50s in a light car with 15x33s and a 540.)
Two of those cars have had LOTS more gear.....(4.56 in the 440 car, 5.14s in the small block) and ran just as hard with less gear.
And you can always adjust things with tire height too.