I think Pep Boys sells Mr. Gasket adapter plates to use those BBC heads though....
Not only do you have a lot of the wrong parts, your car is probably horribly out of tune. You should have someone that really knows Mustangs look at it.
Problem is, every idiot will tell you they know the best way to tune it......hhhmmmmm. I would say pick the brain of someone going fast in a Mustang at the track. Most people are really helpful.
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1988 Mustang GT
12.0 @ 122
1992 BadAzz Wrangler
1993 Explorer