Nice to have you on the forum. I agree with Brett on the group 27 battery. My '77 Excella 500 has dual battery compartments , one on each side of my 40 lb. propane tanks. A grp 27 Delco Voyager 105 amp hr. battery fits fine but the grp 27 Duralast deep cycle was 1/4" too long. According to the Delco site, the M27MF battery is 12.5 inches long, 6.8 inches wide and 8.8 inches in heigth.
'01 2500hd ext. cab, 8.1 litre gas, 5 sp. Allison auto
3.73 rear end
Mag-Hytec rear diff cover
Amsoil Dual by-pass oil filtration system
Amsoil synthetics all around
265 watt AM Solar, Inc. system