i have an amc th-400 with a 10 spline output shaft, i was wondering if its possible to swap in a gm 32 spline and what adapter to use for an atlas 3.8-1 t-case?

You have to disassemble the whole transmission (more or less), then you can change the output shaft.

i know that i was wondering if the diameter was the same and if that would make it harder or easier to find an adapter for the atlas and which one would be cheaper to get an adapter for

Did a swap for a D20 case a few years ago. Afaik the cases are the same, except the bellhousing shape and boltpattern.
The rear bolt pattern is the same on Chevy (4WD) and AMC. You have to take care, there are diff. shaft lengths for the TH400 output shafts.

There where some differences over the years, i cant tell you out of memory which parts where related (imho front pump,...).

For the AA Atlas & adapter take a look on the AA website.

From some research I have done you can pretty much interchange everything inside of the th400 (chevy) to the th400 (amc), and vice versa. So swap away as I am about to do the same. Also for adapters and output shafts look towards stak.....................Kyle