Hey Brett!
Is your advise on the Max III based on experience? I got some on eBay (silly impulse buy, for smallish old-time planes), then found out that they were a Really Hot engine in their day. As you point out, the variation from 15 to 15 is tremendous, I'm wondering where the Max III stacks up.
They weren't a particularly hot engine in their day. Hot engines in their day were the SuperTigre G20/15 and Rossi 15, and depending on how you define "their day" either the Cox Special 15 or the Conquest 15 (which was a copy of the Rossi down to interchangable parts, along with some other copies that apparently exist). The Max III was a nice enough engine by the standards of the day, but that's the sort of engine the "Jr" versions of stunters were designed for. Lew McFarland used to fly his Shark 15 with a Max II or III 15, for instance, and you wouldn't consider putting an 15FP on a 270 square inch airplane, at least for stunt flying purposes - since it will fly regular old 35-size full-bodied stunt planes twice that size fairly well. A Shark 15 does fine with a Fox 15 Slantplug, back before people realized its true purpose. A Max III is in the same ballpark.
Now that everyone is lining up the lynch mobs for me, none of this means that anything is "bad", it just means you need to choose the application appropriately. I flew as many of these "Jr." models as anyone back in the day, the Ringmaster Jr., for example, being my first ever "big" airplane. I had only a G20/.23, it went something like 90 mph, couldn't be balanced properly, and shed parts on almost every flight. If I was building one today, I would use an Enya 09, Hornet 09, etc.
Most of my successful flights with the "Jr" models were with either a Max III 15, OS 20S, or 25S, the 25S being overkill for something like a Jr. Nobler (which, if I recall correctly, showed a Cox Sportsman 15 as one of the alternate engines).
As noted, I haven't ever run my 15LA, much less flown it, so if it is much weaker than a 15FP, then maybe you can be OK with <300 square inches. But I know for sure that this would be a disaster with a 15FP. I would be concerned about creating an uncontrollable "bomb" airplane that would then lead people to want to modify the engine, thus ruining it, simply to make it work like a 60's 15.
Cut and try is a perfectly acceptable engineering method, so trying it and seeing what works and what doesn't would seem to be the way to go, you just need a starting point - 350 square inches and lightly constructed (because it won't have to handle inordinate vibration) is where I would start
Brett