Old ford heads on fox

The answer to your original question is yes you can do that. The push rod motor 289/302/351/5.0/5.8 heads are interchangeable. The big difference is you need to use a larger head bolt on the 351/5.8 heads so you have to tap the block out to accept the larger bolt but this is actually a good thing because it provides you more clamping force with the larger bolt so you don't blow a head gasket. I'm not sure if this is true on the newer 5.0 blocks but on the older blocks they did use a smaller bolt on the head.

As for if the GT40P heads are better, it depends on the head you get. The GT40 head was based on a factory head from I think the 69 351W motor. That was the base design they started with and went from there. I don't remember if it is the 2v head or 4v head but I think it is the 4v head. If you get the right head and port it I think it actually out flows the GT40/P head (even ported) but it has been a while since I've reviewed that info.

If you get a set of GT40P heads just be aware of the spark plug angle, makes changing the plugs a bitch when you have headers on. I'm not sure if he is still doing it because I've been out of the game for a while but Thumper460 used to run a professional porting service and he gave forum members a bit of a discount. Lots of people said he did a great job. If you want to get crazy with modifying the head, you can make a fair amount of power. If you are going to be making 300+ horse pwoer, you should look at swapping the oprange top 19#/hr injectors our for some blue top 24#/hr injectors and get yourself a good tune.