Kind of thinking out loud here:
Sure the battery is installed correctly and not backwards?
What 'size' is the new negative battery cable? Needs to be heavier wire gauge than you think for current to start diesel.
Follow other poster's comments about trying to determine which circuit is pulling load aka shorted. May also need to use a test light or voltmeter. Test light might be easiest---put light between end of disconnected neg cable and chassis ground: light 'on' indicates current flow (caution the light's wire may be relatively small so don't burn it up too by leaving it 'on' when pulling high load).
If EVERYTHING is "off" (proven off, disconnected) then got a short directly back to positive---rubbed raw wire or cable or the two cables somehow touching terminals or connections? Positive terminal touching chassis somehow?
Agree that neg cable may be way undersized due to corrosion or damage. Could be a bad ground connection to chassis creating high resistance current flow point. Try cleaning chassis connection bolt to shiny bright--paint, corrosion, rust, dirty washers, bolt, nut.
If EVERYTHING disconnected and cable okay, then start carefully reconnecting individual components, watching for sparks, and watching negative bat cable. Check for blown fuse(s) along the way after each discrete step reconnecting something.
You should be able to feel glow plugs heating. Could be glow plug controller or timer (in whatever form you have on your machine) failed constantly 'on' and GPs are pulling too much current for a bad ground cable to convey back to battery.
You say only the starter is otherwise off battery. Possible but I think unlikely the starter is constantly energized and not spinning putting huge load on battery, again with undersized-for-whatever-reason neg cable.
Might even be an internally failed battery. Try different battery from another vehicle---not just jumped, but bypassing tractor battery altogether. Had this happen once in the last forty years or so.
Curious what you find. Please post back your experiences so we may all learn.
