The heads are off, swap them or not..

kt340sport said

You might be able to run a J head , but i know you cannot run an X head , i tried it years ago , when i was about 18 .Found out after i run it the block has got to be notched for the large intake valve .The valve actually hits the block .But since the stock valve is 1.78 and the J is 1.80 prob work with the right head gasket. I would just get the original ones done.

Click to expand...

You might be thinking of X heads on a 273 - they will work fine on a 318.

Either way, J or X heads were not being considered because they would result in even lower compression. This isn't going to be a 400hp engine (or let's be honest, probably not even 300). It's probably not going to see much more than 5000 RPM...I don't think it will need tons of flow. If I were building a 318 from scratch I'd use my X heads with a better set of pistons, but that isn't the case here.

The 273 closed chamber heads (which I picked up and the ports appear identical to the 318 heads I pulled off the engine) with a little port clean up and milling should probably do the trick!