[BobbyK]

203 is chain driven instead of gears.
The chain is the weak point of that transfer case.
And yes the NP203's are full time 4 wheel drive. All 4 wheels turning at all time. There are kits that convert them to part time like a NP205.

When coupled to a TH350 is has a 27spline output shaft.
When coupled to the manual Sm465 it has a 10 spline.
And a TH400 would have a 32 spline.


Your SM420 should have a 10 spline output shaft.

I know Advance Adaptors have output shafts to adapt just about anything to anything.

I used them for my 350/700R4/NP205nconversion. It has a 27 spline output shaft.

This pic shows a new Advanced Adaptors output shaft (top one) 700R4 to NP205. The bottom one is the same but broken.