RE: 1/2A contest event
Back in the 1980's Jim Campbell set an altitude record. I believe he used a Maxair Drifter with either a 277 or 377. I want to think he made it into the low 30,000 ft range - I have the article in an ultralight magazine packed away someplace. You just have to either rejet the engines for better operation at that altitude, or use something like the Elison Throttle Body. I'd think your hypothetical B-24 should be able to make into the 30's provided the engines keep running.
Hogflyer