Now that just doesn't make any sense at all and you've got alot of us wondering about your rebuild!
I don't know your tranny or rear setup but a rebuilt HO should allow for much rubber burning in several gears, depending upon your commitment to puncing the pedal and your use of the clutch (if you have a manual tranny). What do you think has caused your issues with low end power?
'68 428 HO M3 Monster, 4-on-the-floor! Need I say more?