You won't have to pull the heads to install a cam. There must be something wrong there somewhere if the car struggles to reach 80 mph. Start with basics to check things out. Compression, vacuum, timig etc. Speed parts won't help much unless the basic engine is OK.
Rev
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'66 Coupe, 306, 350-375 HP, C-4, 13.07 e.t., 104.8 mph, 1/4 mi.
O.B.C. #2
'66 coupe
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