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.)
