I did a little more research and found that Ford didn't use a 5.0 in their mustangs in 80,81. They did in 79, then again. In 82. In 80,81 they used a 4.2. That said your motor if in fact is 1980, probably came out of a truck, or passenger car. Problem is it's probably no more than 160 horsepower stock. You've got a long way to go to make that motor if you use it, to put out anywhere near 300 horsepower. It can be done, but your talking heads, boring, stroking motor, cam. I guess at this point tear it down, if internal block isn't damaged, have block magnafluxed to check for cracks, and build up. I tend to side with the other guys, and buy a quality rebuild. Summit has a pretty good rep. I've had friends but motors from them for Chevys with good results. They have a 300 hp engine for $3600.00, good to go.
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