1984 long bed 22R carburated...

I'm getting a strange variation when I measure battery voltage using different ground points. I use the battery positive for the red cable from my DVM. For the black cable, if I use the neg battery terminal I read 12.3v. If I use the battery cable where it bolts to the passenger side engine compartment wall, I get 12.3v. But if I use a fender bolt on the driver's side engine compartment wall, the reading is 10.9v. If I use a bolt head screwed into the engine itself, the reading drops to 8.8v . Same low number if I clip the neg DMV cable to a metal brake line.

There may be a ground strap from the engine or trans to the chassis that has a bad connection (is there one on this car?), but I don't know why I'm getting a voltage drop from one side of the engine compartment to the other side. Both bolts are tight. Is there something I need to know about this truck?

John