Thanks for the detailed response...Here's what I found out in the last several days.

All positive connections (at the starter, at the alternator) read 12.5 volts between + and body ground. Once the jumper is connected from the block/starter ground point to a body ground, the lights work and the vehicle has electrical power.

Checking grounds I found the following:
Alternator -ve to body is 0 volts
Battery -ve to body is 0 volts
Body to block is 0 volts (with ignition off, haven't checked with ignition on but will do today)

The problem is between -ve battery and the block/starter ground point. If I run the jumper cable between the bolt at the block ground to a bolt on the body, the vehicle gets power but the starter barely runs (tries to crank isn't getting sufficient current). If I run the jumper directly from battery -ve to body, the vehicle gets power but the starter doesn't run at all. If I run the jumper from block/starter ground to body and carefully place the jumper such that it's touching the loop at the block end of the -ve battery cable, the car has power and starts right up. It runs for ~1 minute then loses electrical power (battery reads 13.5 volts across the terminals while the engine is running, so the alternator is working). So the ground at the block/starter is bad - I need to check how rusty it is.

Other than rust interfering with the -ve battery cable connection, how could the block be electrically charged? A short across the starter?

I did try jumping it originally with + to + and - to -, something I never do but I was in a hurry and it was pitch dark out, I found out the next morning the -ve battery cable was severed due to corrosion (close to the block/starter ground point) and I replaced it with a new one.

Thanks again -
Zack
1980 245DL M46 286k
1988 745T+ M46 224k