The 81-83 N/A 5sp box main shaft nut is threaded opposite from the main shaft nut on all the other z-car transmissions. It was done to prevent the main shaft nut from loosening as the tranny aged. I have opened and repaired many 4 and 5 speed datsun Z boxes, and to a "T" I would either recommend staying with the "B" type 4sp or install a 81-83 N/A 5sp. The 77-79 5sp was a bastard box....essentially a 4sp that was converted and or modified to make it a 5sp. The 81-83 5sp was a purpose built (to last) 5sp gearbox.
As far as your issue goes, your problem is prolly not tranny related at all, but clutch, and for the record...the quickest, as in 0-60 time, L-6 powered Z I ever drove was Turboed with a 4sp and 3.54.