The 400 likes a longer duration cam than a 350 would. I would suggest something in the range of a 274 advertixed duration for a mild street motor.

You can spend a lot upgrading the stock cylinder heads with big valves and screw in studs. Once you've installed the large valves you need to have the heads hand ported to allow the valves to flow properly. Just putting 2.02 valves in a head that had 1.94's without any port work will give you less flow than stock due to valve shrouding. When you are all done you will still have a set of crack prone heads that don't flow very well. Better, and likely cheaper in the long run, would be a set of aftermarket heads. Dart offers both steel and aluminum heads with 72c.c. chambers.