According to the DD Field Service Data manual, 6V92 is recommended to have 2-205amphour batteries in parallel or 1-410 amp hour battery, for a 12V setup.
The group 31's are usually rated between 95 and 125 AH, and the 8D is rated at 225 - 255 Ah.
Interestingly enough, the 4D seems to fall right into the lower end of the requirements at 180 to 215 AH and is pretty easy to move around.
I use 2 group 31's for my start batteries, buut, then too, I'm in South Carolina, so cold starting isn't an issue. If it became an issue, I could kick in the other 6 31's to give it a boost.
By the way, your 6V71 IS a GMC engine.
The information quoted here comes from a number of websites and other places, one of which you may find interesting:http://www.ccis.com/home/mnemeth/12volt/12volt.htm