yes I plan on doing all the work myself, i just want a project bike/ caferacer, with a small fram and bigger engine. I was hoping it was possible to shoehorn a 750 into a 500 frame but it is looking doubtful.
doubtful? There is less fabrication in building a new frame than there is in adapting a 500/550 to take a 750.
The big engine little frame is a modern sport bike concept. Most old bikes use heavy steel tubular frames and really there is no weight savings unless you are putting a really big engine into a really small frame (say a cb750 into a cb400). Most of the power differences between models (espically hondas) can be made up with good tuning and a few speed parts (e.g. finding 10 hp in a 550 motor will get you to the 58hp most midyear cb750s put out stock).
Of course there are exceptions and you may want to consider other brands and two strokes. For example a kawasaki h1 500 or h2 750 two stroke in an s3 400 frame has a significant weight reduction and is possible (a 750 in a 500 frame is even easier since the motor mounts are the same, but it actually adds weight). Most two strokes can be made significantly lighter than their 4 stroke counterparts. S3 400s are cheap if the motor has a problem since pistons are scarce and 500 motors are pretty common and have a lot of speed parts available. 750 motors are out there but pricey.
read about a 500 into 400 frame here:
This bike belongs to cody (scrambler73 on the kawasakitriplesworldwide.com message board). It weights about 360 lbs and is a 750 in a 500 frame. A more badass road burner I have yet to see.