Well, now that thats out of the way, clean your battery to frame ground point, and check your hot wire from the battery to the solenoid, and from the solenoid to the starter for damage.
One way I check for resistance in the field is to pull the plug wires off, crank the engine for 10 seconds...then start at the battert and feel all the wires to their termination points for hot spots. a Hot spot indicates increased resistance, which is basically bleeding off amperage.
If the battery terminals, ground, wires, solenoid, etc arent hot after say 20 seconds of cranking (10s 1 minute cool down, then 10s more) then go to the starter...is IT hot at the hot wire, or is the case hot, lower cap hot?
These starters and wiring systems were just adequate to crank over these twins new...any loss of efficiency can make problems.
Also a dragging PTO belt can make problems too.
I had a little time to look at this tonight. Just starting it,,rolls very slow at first,,then will take off. Just doing this ,,the neg cable is getting hot. I removed battery,,and cleaned the connections,,to battery and to frame under battery. Didn't help!!
The cable looks good. It has a small wire going from the connection at frame ,,to 25 amp fuse,,then to solenoid.
Positive cable is not getting hot at all.
No belts are moving with brake applied,,so can eliminate that,,,I think?? Not sure if there could still be some resistance..
Do you think this short negative cable is the problem?? Or could it be something more??