I'd do a compression test. My friend had a 84 GT Mustang with a weird combination of 89 LTD 302 and some newer HO parts. I think he tried to run a carb on some of the new HO fuel injected heads and I think he had problems too. I think that those heads will work much better with fuel injection because of port angles, breathability, and the compression ratio you get from using them. But I could be wrong I know more about Chevy.