I have been trying to gather some information on a transmission swap, and how feasible it would be. I currently have a 74 K5 with a 350, SM465 and a np205 tcase. From what I have read, they are the older models, with the figure 8 bolt pattern. I was thinking of swapping an nv4500 for the overdrive, however, I believe that matches up with the 6 bolt circular pattern and 32 spline input t case. I would need an adapter then, correct.
Does all of that sound right?
Additionally, I know there are changes required for the clutch, either to convert to hydraulic or use an adapter to keep mechanical.
Anyone who has had experience with this kind of swap, let me know how you have done it.
I have pictures of stamp codes if that will help also.

 

Lots of vehicles got lousy mileage in the 1970s due to low-compression motors and restrictive exhaust due to the lousy Catalytic converters. K5 Blazers were no exception. Might think about raising the axle ratios instead. Personally I would leave it stock or sell it. The NP 205 has a granny gear, the NV3500 does not.