The bumper is a square body 73-87 bumper not sure what year exactly, and you said he had 75 step side with it. i also noticed the power steering. Any chance that's a 75 frame with a 60' body on it? a pic of the back frame rail could help solve. almost looks like the rear cab mounts are c10 mounts bolted to the frame. I thought c20's had different ones.(could have been swapped either way)
not ment to rain on your parade just my observations.
by the way it does look like a great start.
