400's are a little bit tricky to build. If you want to build one with the stock bottom end that is fine, as long as you don't plan on reving the thing past 5500 rpm's. The short connecting rod length is the problem. They are made for making torque, not high rpm's. You can change this, but it requires new connecting rods(5.7 or 6.0 length), and custom pistons. You're getting past the budget build at this point. I would do this if you plan to rebuild the entire engine and have a couple grand to throw in it.

If you want to get new heads, and keep the stock rods and pistons, get some small heads... something like a 165-170cc intake runner. And put a small cam in it, no bigger than 220 duration @ .050. You'll want a rev limiter as well. Don't rev it past 5200 rpm's and it should be fairly reliable.