Whenever I have an odd reading on a factory gauge, I'll use a second gauge to verify. For oil pressure "T" in a mechanical gauge, for engine temp an IR temp gun, or drop an old fashion ether gauge down the radiator fill, if one exists.
Does it have factory cruise control?
Most likely the plastic gear at the transmission is stripped OR the cable is broken. You could disconnect the cable from the trans and spin it with a drill, if the needle moves it's a gear problem.
I put a vintage air system about 10 to 15 years ago in my '85 CUCV blazer. The blazer has a lot more interior volume than a regular cab, so the a/c was ok in the front but not that well for the rear seating. Realize the CUCV's had no insulation or sound deadening and was heat absorbing dark green.
In for a dime, in for a dollar. Looks like a blown head gasket. If it was mine, I'd pull the nose and get a Harbor Freight engine hoist for ease of removal.
With something like a turkey baster and the cap & rotor off you can pull a vacuum and see if the advance plate moves and holds.
Any idea what is your axle ratio?
Is it getting gas? That fuel filter looks like it may be the culprit. Hook up the choke to a fused 12 volts when ignition on and get a new rubber plug. Don't forget to ground one side of the choke.
You are having the same voltage drop issue that I have with my '74, however my truck started with a points distributor and the previous owner installed an HEI. To get mine to run I need it to get it spinning (key in start position) then let it go so it is in the run position. When I get around...
I don't know about the different years, but I've put a mid 80's C10 cab on a mid 80's K30 chassis. Both were column shifted automatics. I just needed to cut the tunnel for the transfer case shifter.