1986 Kenworth 3406b starter wiring

The signal is on the small terminal on the opposite side of the solenoid from your small ground.

There has to be a signal to that small terminal. If you jump from the positive solenoid stud to that signal it should crank if your grounds are correct

 

Should be a smaller wire going to the starter solenoid from a relay or a mag switch like the one you posted pics of in the battery box.

 

Here ya go for placement. G would be towards the block, S towards frame rail
SB is starter body
P is Positive
S is Signal
G is ground

 

See if this helps, may not be exactly what you have. "came home and threw them on in the dark" Best to slow/trickle charge before installing batteries. I have had them charge for 4-8 hours even with a mfg date of only a month old. Not that this is your problem.
This is a good video even if it is on an F-150.
Ford F-150 No Crank No Start: Engine Starter Motor Diagnostic Walkthrough! - YouTube

 

This looks like it would be the signal wire, and that exact spot the arrow points to looks compromised. I mean, that looks like a bad spot to me. I just blew up the pic you posted

 

Often running a new wire is best. Find the relay thatvis clicking when you hit the button. That should be where it needs to go

 

Many Kw’s in the 80’s were still positive ground. Do’s yours look like it may have been? Alternators were different polarity.