The heads are off, swap them or not..

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.