It is a 2nd Series which makes it like the 56-59 trucks. Being a 6100 it has the wide grille/fenders & they have large wheel openings.
I put it on my old trail rig 76 K20 chassis. It's GM Corp 10 bolt front & 14FF rear. SM465/NP205 trans/case with a 350 engine.
Here's what I started with years...