Are the heads on an hp500 (carb motor) the same as a 502 mag motor? Could I use stock 502 Mag heads on a stock hp500?

Nope. 2.19/1.75(?) stock. 2.25/1.88 hp series. Valve sizes.

jason is right. plus i think the hp series is inconel .stock is stainless they will work though

Is the hp500 carb motor gen 4?

I though the 990 and 188 heads had 2.19 intake and 1.88 exhaust. So with that I thought only the intake was bigger.

Yes the hp500 is a carb motor.

I have a stock hp500 with a couple leaking exhaust valves. It is a spare motor that I bought from a friend that didnt need it. I guess I bought it more as a spare motor than anything. I thought I would use it while I put a 540ci motor together.

I thought maybe I could take the 502 Mag heads and EFI system and set it on the hp500 rotating assembly so I could have at least a motor to use in the mean time. I thought I would use the EFI set up vs. the carb because I am in and out of Denver Colorado, Oklahoma and Havasu and didnt want to have to worry about carb jetting and stuff. Thats what I was thinking anyway. If I am off track, let me know.