I should have clarified, I did mean a 450 four stroke motor in a 250 four stroke aluminum chassis. I'm posting this for a friend of mine and he''s told me that there are very few 450 aluminum frames floating around and the ones that are are 3-4 times the cost of a 250 frame. I was talking to him about the conversion and he seemed to think that aside from the frame, subframe, rear suspension linkage, and possible the motor mounts everything else should bolt up. I wanted to see if anyone else had attempted this mod either with a 450 or 250 aluminum frame with an older motor to see if they could verify the parts bolting up and whether or not it's worth the headaches. His current 03 motor has been completely rebuilt but his frame and subframe have taken 300 or so hours of abuse and while he loves the motor he figured he could drop a few pounds and put himself in a fresher chassis.
Thank You to everyone that's posted.