the way the gear sticks fit in the boxes is different and im pretty sure not interchangable that is one way of telling besides the obvious of length

R160s are pretty common still, but the R180s are a little thin on the ground. not many ratios available either, it's pretty much a 3.7:1 or 4.11:1 now. i'm sure they're out there, just needs a little time for looking.

as for the gearboxes, not too sure. sorry.

All A-series rwd gearboxes are nearly exactly the same length.

Here's how you can inspect a used gearbox that's out of the car:
1. Move shifter through all the gears. Look for smooth operation, no surprises
2. Try to put some resistance on the output shaft, and turn the input shaft by hand. There should be no funny noises. It should turn smoothly
3. Inspect for any signs of damage to the front and rear splines
4. Tip the gearbox forwards and back (just a wee bit). A little gearbox oil should come out, but NO WATER. Avoid it if there is any water.

hello people. i have some gearbox questions. firstly how do i tell the difference between a datsun short and a datsun long gearbox? i think this is a 1600 thing but hopefully someone knows. secondly how do i tell if a gearbox is any good just from a visual inspection (its not in the car) is this possible? and finally whats the availability of the R160 and R180 diffs like? Thanks