It all sort of depends upon where the battery(s) were located.
If under the hood, most likely a single group 31 battery.
If in a battery compartment located in the skirt of the body along the frame rail it could have had one large 8D, or two/three Group 31 batteries, or two 8D batteries. I have seen some skirt mounted battery boxes that were just that, boxes. In that case changing the size of the battery or number of batteries is not a big deal. Others I have seen have had racks that will only fit a certain size battery. Regardless, choose whichever will fit the best.
For your purposes you don't need much more battery than one high capacity Group 31 or one 8D battery. Most buses have multiple batteries for the reserve capacity. When you have six or more heaters, defrosters, and blowers in addition to crossover lights, interior lights, running lights, headlights, and roof strobe lights it takes a lot of juice to keep them all going. Even with 150+ amp alternators it is hard to generate enough juice at idle to not use more than what is being generated. Since you will be doing very little idling with much more than a few lights you won't need the reserve capacity for that and one battery should be more than enough to get your engine started.
Whichever battery(s) your bus had, make sure you use large enough battery cables to reduce the resistance from the battery(s) to the starter. That starter is going to draw a LOT of amps whenever it is turning.
Also, don't purchase battery(s) strictly on price. When you shop around make sure you are comparing apples to apples. I have seen some Group 31 batteries that have had more cold cranking amps and reserve capacity than some 8D batteries.
