If spark timing and compression are found to be good, your next next option is cam timing. Your camshaft timing may be retarded either through installation error (pretty unlikely) or manufacturing error.
As a kid, I installed a Mellings 300 degrees adv duration .512" lift HFT cam in a Ford FE GT390 "by the book", dot to dot. In those days cam degreeing was a black art left to engine wizards. I wound up with a very disappointing motor that couldn't get out of it's own way under 3,000 rpm. I had the distributor converted to full mechanical advance and changed to a 3.5 power valve in the Holley 3310 to keep it from flooding under low vacuum.
Even with those changes, it was a dog on the low end with a huge stumble on clutch engagement under normal driving. Once it hit 3,000 rpm it was like you turned on the "power switch". At the suggestion of the shop who sold me the cam, I advanced it 6 degrees with an offset key and it really woke the car up. The difference was like night and day.
You may be in the same boat.
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Triple Black 1971 GTO
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