You don't need a huge tank per se. What you need is the engine to start promptly. That would work for a hot restart, but I'd question it for a cold start. The tank on Mack trucks from memory is probably about 20-30 gallons and cranking time, even cold, is about 2 seconds. So you'd use your pony to start cold and if you had no air in the tank. You could use the air start for a hot start. You could run a generic Bendix compressor on the D315 to recharge the tank just like trucks do.

If you didn't need to be all fancy with a key start solenoid valve, you could just use a regular valve like those rapid tire inflater tanks. Need about a 2" dia pipe from the tank to the starter motor. Rig up a longer belt (vbeltsupply.com) for the Bendix, add a D2 unloader and you're cooking with gas. Need video!

You said it Iron Beaver - finding the air starter that fits. You'll easily find one that fits in the hole, but pinion fitment might be a different matter. For electric starts, the 11 or 12 tooth helical (can't remember if it's CW or CCW rotation) is what's used and it's an assembly predicated on solenoid engagement. Your air starter as mentioned previously is likely to be inertial engagement. Of course anything can be made so if you "really" wanted to do this, that's where I'd start on figuring out the logistics, because that's the hardest part I think. You already have a compressor, generic valves and tubing are available anywhere, probably pickup a tank or two from a Mack dismantler, and an air starter from ebay or somewhere so everything else seems fairly straightforward.