I've only used the brackets on the front & rear diagonal corners of the intake manifold. Sometimes using the end of the head or exhaust can mangle up the valve cover.
I forget if it was my old 6.2NA or 6.5TD that use to eat starters, I ended up extending the wires and used a weather-pak connection for ease of removal.
Was it running hot before you messed with it? Did the replacement sending unit look like the old one? Could have mistakenly installed a sender for an idiot light?
A plus on pulling the cover is you can see if the gear oil has any water in it and can check for any sparkles in the fluid. You'll only be out some silicone sealant and 2 or 3 qts. of gear oil. If you've never smelt gear oil, if it smells bad, it's good.
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?