I'm gonna say you'll have a hard time maintaining any particular altitude if you're dead set on flying through thunderstorms. Request a block and maintain level flight with your, I dunno Chuck, friggin carb heat on or your inertial separators open? Penetrate at your turbulence penetration speed at the whitest, narrowest, least puffy-looking area as far upwind as you can get. With no 02 I'd stick at 10,000 so as to, ya know, maintain the flow of oxygenated blood to your pea brain.
This is all assuming that for some reason prior to reaching your point of no return you had been listening out on 123.45 and 121.5 for PIREPS and canvassing other aircraft on your route of flight about conditions down route. Had you heard about developing thunderstorms a decision to turn back might be smartest.