Can you look at the transmission hump? Can you slide your hand along the side and feel for some bolts? The 80's transmission humps were smooth and not bolted in. The seventies were bolted in.
You should be able to read a serial number on the dash by looking through the windshield on the drivers side. It might have a aftermarket pad installed IDK.
I think you should take a close look at the rear cross member by the bumper. I think the 80's trucks have large holes drilled through the center of them. The 70's do not. This will tell you if it is an 80's frame.
I have an 83 with a bolt in hump. Also 83 and down had the wipers on the dash. Most likely an 81'-83' with a pre 80' dash.
All 73-87 frames are basically identical except for the front bumper horns. The 73-80 are rounded and the 81-87 are cut off straight at a bit of an angle. This is really the only way to determine which era of frame you have.
A pre 80' cab would have to have the hinge openings cut into the cowl area so if it doesn't look factory there, then you know.
It is difficult to retro an 81-87 front clip onto a 73-80 cab. But easy to go the other way.
Odds are...especially if it had a VIN under the dash that it is an 81-83 truck with a 73-80 dash