This is interesting. I happen to have an 09 250r and a 91 ex500. I've read that a stock ex500 motor can twist a stock ex500 frame when pushed hard at the track. So the 250r frame might not be that bad. I'll have to look the two carefully.
I'll probably still just rebuild my 500 separately anyway. I actually like the 250 the way it is, after the needle shim and snorkel removal that is. But if I can find a cheap 250r frame for a build out, I may put the 500 in that. Save on insurance
Bob