361 heads

If it's a mild to moderate 400, there's absolutely no reason in the world the 516? casting heads will not work.

But, as already mentioned, they will need hardened exhaust seats. But what else will they need? 361s have been gone a LONG time so the heads have to be pretty old and pretty used. If they need guide work too, that will be a pretty expensive set of cast iron heads.

The shop I use here would get around 400 for guides and hardened seats. That's a good deal compared to most. Add to that whatever valve job you want.....a good competition valve job is usually around 200, milling, cleaning and magnafluxing and you have a pretty big bill for some old used cast iron heads.

Put a little more with it and you can have some Stealth heads. Of course, keep in mind even those shouldn't be bolted on out of the box. So you will have some money in getting those checked out and ready to run.