The later 302's and 351's and exploder motors had different firing orders. Supposedly it was done to make the engine run cooler. I found this out the hard way, my 73 was rebuilt by a friend years ago and when I went to start it the thing popped and banged and ran like crap. After checking everything and re-stabbing the dizzy a dozen times I couldn't figure it out. One night it hit me, maybe the RV cam we put in has a different firing order. Looked up the late model Mustang firing order, swapped the plugs and it ran perfect! Give it a try, might just be the issue.