1755 battery cables,, glow plug cable ?

Have a 1755 that I am putting in 2 group 31 12 volt batteries and new 2/0 cables. Have not used or tried to use the glow plug for years. Not really sure it even works. Looks like cable goes from starter to relay ? Then to one glow plug ? Thinking I could just leave the cable from the starter out. This is a chore tractor that gets plugged in all winter. Also putting gear reduction starter on. Think I probably don't need the glow plug. ?? Thoughts ?

1 Gage would be sufficient, 2/0 is way over kill.

Personally I would hook up the "Glow Plug" or the Manifold Preheater system. The warm air that it introduces into the intake is a great help, even when starting in temps of 30's & 40's even 50's. It is a very simple system and easy to repair and diagnose when not working. You are correct there is one cable from the positive post on the starter motor to one side of the relay/solenoid then another short cable from the other side of the relay to the actual element. There should be a push button switch on the dash board to energize the relay with key on. There is a smaller wire to the relay also it comes from the push button switch and another wire from the key switch to the push button switch. All of these components can be checked with a volt/ohm meter just by checking continuity of each part and checking for 12 volts to the relay from the battery and then at the element when the switch is engaged. In my experience with these systems is either the relay is bad or the element is not working. Most of the time with the element I have found the coils are broken and can be repaired by solder or welded.
You are defiantly doing a good thing by installing the new 31 series batteries and the new starter. That alone will make a huge difference, the faster you can turn over a cold diesel engine the faster it makes heat and will help it start. Make sure you have good cables and nice clean ground connections. Plugging it in will be the most help of all. All these systems together and working properly will help your tractor start in about any weather and every time you want to use it. Hope this helps and or answers your questions.