I did it about 2 months ago.. pretty easy considering.
I swapped a TH350,NP203 to a SM465,NP203,NP205.
Almost everything lines up the same.
Issues I've had are:
-Hydraulic clutch from mechanical
-Front drive shaft lengthened (due to double T-case setup)
-I had 2 cross members and the primary one fit just fine, but the second one required a little grinding. (also due to double t-case setup)
-Custom shifter for 2 T-cases
That's it..
As far as cost, depends what you can find.
A used transmission will probably run you about $150
Same with a transfer case
Then it depends if you want to rebuild the items or not, rebuild kits are about $100
The hydraulic clutch cost me about $150
Of course the adapter to do dual t-cases is probably the most expensive.
So the moral of the story is if you don't want or need the dual t-cases or a hydraulic clutch you can do it quite easily and cheap.
Adam
'75 383,465,203,205,1/2ton axles (I'm workin on it), 35's