Trying to figure out what chassis my truck is on.

[Outlaw_71]

Truck is on a 4wd chassis. I believe the t-case is np203 since the shift pattern is-

(front)
low loc
low
nuetral
high
high loc
(seat)

and this is on the knob as well.
It has a 350 sbc. I'm not sure whether the engine is from the truck the cab and bed came from or the truck the chassis and trans/transfer case came from. The reason i know it is not the stock chassis is because the vin on the glovebox says it was an original 2wd truck and the t-case shifter was brought up through a cut through the floor boards. I guess my question is what years chevy fullsizes had NP203's with this shift pattern and how to determine what year the chassis is. My mechanic geustimated about a '75 silverado. What can i check to identify this?

EDIT- If it helps it also has locking hubs and i think my mechanic said it has a 350 trans. It also has heavy leaf packs. I'll count them tomorrow.