I might be wrong, but I believe that bare wire is actually the positive cable going from the solenoid to the actual starter motor (connected internally). So putting a ground there would be a very very bad idea.

The solenoid will ground through its case to the starter, which in turn grounds through its case to the transmission adapter/spacer, which in turn grounds to the engine block, which is grounded to the battery by the main ground wires.

Even though those are all pretty solid metal to metal connections, it might not hurt running a separate ground wire direct to the starter, but I'd run it to one of the mounting bolts if I was going that route.

 

I crimp, solder, and heat-shrink whenever possible. I'd like to KNOW my electrical connections are not an issue in about 5 years when I'm diagnosing some other issue. I may have to pull a ring terminal and clean the stud of corroson, but my wire/connector joint will be good. I pull the blue or yellow plastic crap off the solderless connectors, crimp the wire as tight as I can with vise-grips, lay a bit of solder on them, then cover with heat-shrink tubing. For the battery cables, I used a propane torch to solder the brass lugs on the end, then covered with heat-shrink tubing.



I LOVE my marine terminals. The were actually recommended to me by the guy at Carquest. Since my cables were in good shape and only needed fresh terminals, I went with the marine terminals and brass lugs to address the passenger side battery with the 3 cables all coming together at the terminal.

 

The hydrogen sulfide gas is what causes the corrosion inside the cables.

A nice coat of liquid tape over the cable to connector joint, heat shrink, more liquid tape over the ends of the heat shrink to make sure they are and stay sealed.

2/0 for the cross over cable, 3/0 from the passenger side battery down to the starter.

The starter relay mounted on the fender sends power down to the starter mounted solenoid small terminal.

Both battery grounds go to the engine block.
One of them should have a second wire on the lower end to ground the frame.
And a second ground wire on the top end should go to the body.( I actually have a body ground for each battery and like it, but I have a ton of electrical stuff on mine.)

Glow plug wiring harness from the relay to the engine block, leave that wire size as it is.
It helps reduce the voltage to the 6 volt glow plugs.
Power from the battery to the relay, a bigger wire is OK.

Inspect both connectors in this picture.



Pay close attention to the end with the large AWG orange/white wires on the one circled in red, glow plug power wires.
If it looks like the connector has been hot, melted looking, splice the orange wires arond the connector.

If the yellow circled connector looks melted, splice the yellow wire together arounf that one as well.
The one circled in yellow is the main power to the ignition switch.

Your original problem very well could have been the clutch interlock, down by the clutch pedal look for a connector with two red/lt. blue striped wires.
One wire is the start wire from the ignition switch, the other is the wire running to the start terminal of the starter relay mounted on the fender.

 

Checked the resistance between that bare wire between the solenoid and starter and the body of the starter - 0 ohms. Just to be on the safe side I'll stick with running a ground to one of the lower mounting bolts - I'm sure getting that top bolt on will be hard enough as is without having an extra 3/0 wire in the way.

Took a closer look at the small wire from the relay down to the solenoid and realized it was barely holding on by a few small strands underneath all the crud at the crimped connection on the solenoid side. Hopefully that was the culprit but thanks for the advice Dave, will be checking all the wires you mentioned. By the way, are the blue wires part of a manual glow plug system? Kinda hard to tell from the angle of the picture but the splice into the purple wire near the bottom of the picture makes me think so. By coincidence I used blue wires to run my push button set up.