There are a bunch of things that can make a motor not accelerate well like to rich at part throttle, valve lifters with not enough preload or to much preload, degreeing the cam off of the exhaust lifter instead of the intake lifter , not enough advance at low RPM on the ignition timing, to cold on the heat range on the spark plugs and probably some others that aren't coming to mind right now
How tall are your rear tires now?
Is your NP 833 a 2.65 low gear set or is it the later 2.2? low gear set As already suggested using a lower first gear ratio like the stock 3.09 low gear set makes a bunch of difference on the street
Been there done that years ago when the early A body trans. where plentiful in our local SO CA junk yards
I put one of those gear sets in a friend of mines 1970 Duster 340 4 speed street race car that ended up with a set of 5.57 rear gears in the Dana 60 , you needed to be on your game on the first three gear changes in that rascal It flat hooked and book with a set of 13x30x15 drag slicks on a set of 12 inch rims
It showed a bunch of BB Chevy the tail lights on lots of occasions