I searched and did not find what I was looking for.

Im building a K5 on rockwells and 44s. 350 motor and a turbo 350 tranny.

I have been given a free NP 203 tcase.

My question is should I run the 203 or should I hunt down a 205. Which one is better? Either way I have to find a adapter because its a 2wd turbo 350. Second question is will the adapter fit the 203 and 205 or is there 2 different adapters for both cases?

And no i dont want to do a doubler.

different for 203 and 205 i believe. 205 is 100X better. and with a 2wd th350 i believe your gonna need a different output on it. shorter i think?

The 203 and 205 take different adaptors but use the same output from the t350

go ahead and hunt down the 205 and make a 203/205 doubler

The 205 is stronger, lighter and shorter.

What is the input spline count on the 205 you have? Should be 27 or 10 spline.

10 spline would go behind an SM 465 and a 27 will be a th 350.
Depending on the output spline count you may have to change the input of your 205.

The TH350 will need the rear ouput changed to run a tc, unless you run a divorced unit. With a K5 you dont have enough room for that. If you go the 350 route you need the adapter, change the output and ditch the tail housing. You would be best off finding yourself a 4wd trans and running the 205. I think Novak makes an adapter for the 350/205 due their being hard to come by. All GM 1980 1/2 tons that had an automatic had a th350/205 set up but were slip yoke (thats easily changed out).

While I think alot of people dump on the 203 needlessly, given your application I'd go with a 205. The 203 is plenty strong for most applications despite what you hear, but since you've already got the huge axles, why not take the Xfer case out as the weak link?