Does anyone know if all FE motor cylinder heads are interchangeable? I've heard so but I want to make sure. I've got a 352 and I'd like to put 390 HO heads on instead of the factory ones.

 

Ask a machinist at your local speed shop to confirm the following.
Years ago, I built a 352 with great result. The 352 is considered to be a high winding engine because of it's bore stroke ratio like the venerable 427.
I used the 352 heads I had with the engine and they worked fine.
I recall that to put the 390, 428 heads on your engine you need to champher the top of the cylinder bore to get clearance for the larger valves with the smaller bore. You can get a really great flow from the 352 heads you have with minimal porting and getting a roller cam.
Hope this helps