Final drive is the same on all boxes as it's a straight through connection. There are different ratios on the overdrives, with the highest being 28% on a J type overdrive - if you can find one, but they are usually 25%. As far as I can make out 2nd and 3rd gear ratios are all the same across the range, but the vans have a lower first gear than the cars.
The problem with the arrows cars is that the input shaft is longer than the commer boxes. You can do a mix and match of the box, but it involves dismantling both boxes, as you will need to combine the gears and output shaft from the OD box with the input shaft and bell housing from the commer. Not impossible, and not the most complex thing to do, but you will need a couple if very large spanners (1 1/2" and 1 5/8" from memory).
Hope that helps
A
A wandering commer blog - http://commerblog.wordpress.com/