Bingo! My bet is that exhaust is a major problem there. If yours is the older GT-80 with the stock steel mufflers, it would probably only turn a 24-10 in the 6200 - 6400 range at best and then adding those extensions will further restrict its performance. You might want to do a test run with the mufflers removed just to get a baseline RPM reading then go from there. You need to prove to yourself the engine is good even if it is new. At minimum, try a test run with those extensions removed. You'll be surprised. That engine does not like exhaust restriction.

The older stock mufflers are horribly restrictive themselves!