Sounds like you got a sweet truck! I have small 24-volt solar on mine to keep the batteries charged up. Having a wire get hot when you put it under load can be frustrating. Think of the 24 volt starter power as a circle. You have the batteries and the one cable that hooks them together...then the short positive wire to the firewall block...which you say is the one getting hot..then a big wire from the block to the starter. If that last wire isn't getting hot the block is the issue...with one side note. Take a look at the block and it's connecting bar inside. I like all of my trucks to share the stud that has the short battery wire and the long starter wire to the starter. Get those 2 wires on the same stud. I like to take the connecting bar out and clean it up and use star washers on each nut. Test to see if the wires still get hot. Here's the side note...the main ground from the block to the intake...its something I've found loose a bunch. It is a stud that holds the intake down with a nut to bolt the wire to. Gotta take the stud out and clean the threads and torqued it in place..pop a fresh lock washer under it too. I've found the actual stud is loose, but the nut is tight, making you think the wire has a good ground when it doesn't. Torque the nut on that cable. That's the other side of the circle...I also like that ground wire to share the same stud on the block and the main ground wire coming from the front battery. So after all this, if the starter wires are...both... still gets hot, the starter is the issue. The load is wrong...my guess would be a 12 volt starter was put in, or the starter bushings are worn out. Make sure starter bolts, battery connection, and front bracket is tight.
Just for reference, the block is where all the power and ground wires are connected to on the firewall...not the engine. Last thing is the frame. It only needs a small ground for the fuel level sensor and rear tail lights, so a big wire isn't needed. I am assuming batteries are good, not just charged up, and both alternators are working. See what you find and post up what you find with pics of the block, wires and batteries for fun.
Sent from my SM-S906U using Tapatalk