400hp N/A is going to cost a bit of money no matter what you do. I would suggest the 351 because it will be far easier to get 400hp with the 351 than the 302. The 351 will be far more streetable at that power level.

Basically, you can spend your money on big heads, cam, intake for a high strung 302 or spend your money on swap parts for the 351 and have a good start towards 600hp if you decide to later.