Hey guys I'm new to this page was wondering if I could get sum help on here I have a 1994 f150 that had a 300 i6 in it I swapped it for a 302 5.0 out of a 1991 mustang but quickly found out that I was not going to be able to use that harness so I went to the junk yard pulled out a harness form a 1994 f150 with the 5.0 in it but I haven't been able to get it to start any ideas tried the computer from the f150 5.0 and the computer from the mustang also the guy before me that had the 5.0 installed a cam,lifters and rocker arms thanks in advance

 

The 95 and 96 F150's had the HO cam, which is a different firing order from the standard 302 firing order. Most of the 94's had the standard firing order. And I could only guess which one the mustang had in it. So there could be an issue there.
Also the computer controls not only the engine but also the transmission so if the transmission doesn't match with what the computer came out of that could cause issues.
But start with the basics - fuel pressure (use a gauge), spark and timing. After you verify that you have the firing order correct of course.