Thinking I had drawn a diagram of all the wires before pulling the transmission, I seem to have neglected the large black ground wire which is now resting on top of the trans. Where does this one attach?

There should be a 6mm (or is it 8mm?) stud extending from the bottom side of the trunk floor, toward the mid/aft end of the transaxle.

If not, anywhere on the chassis will do.

EDIT: BTW, while any good wire will do, the stock one was a braided steel strap.

Thanks, Greg. I've got the braided ground wire hooked up. This one is a round, battery-like cable coming from above the passenger side. I'll have to trace it to its origin and figure out more from that.

The only large cable on my 75 coming from the passenger side is the power supply lead for the starter.

It is black...

But it is not a ground.

What do you have hooked to the large terminal on your starter?

Thanks, tech. That's got to be the answer.