what heads to run on a 273

Not exactly but pretty close. The only real advantage of the 302's are the hard seats, and in my experience, a little better gas mileage. If you have the 302's already, sure, use them, but you'll have to decide if it's worth the expense to buy some for those minor advantages.

Exactly. The 302 heads aren't going to make a 273 a lot stronger or faster.
If and when you tear down a 273 for rebuild, you find valves recessed into the heads or other issues, The 302 casting is the most practical way to renew the engine to at least its original specs.