AS far as I know there was 2 types of 6 speed gearboxes (manual ones) 1 was with highway 6th speed that dropped your rpm, the 2nd one is shorter on 6th gear.In fact my BKD on 70mph 6th gear got 2200rpm.
Seems we have different gearboxes then as you stated because my BKD stays at 2000rpm on 70mph. Do you know of any differences between these, the gear ratios, etc.? Seems that mine isn't that reliable as I already had to hack-fix it because it went bad (would get stuck in 1st and R and jumps around when I turn the car off).
EDIT: Just found this on another forum, all of these are for BKD engine:
GNE/HDU/JMA all use a 70/19/24 (3.68) diff but listing 2 different part numbers
GRF uses a 69/20/25 (3.45) diff