imo i would say its not worth it, you would bebetter off selling the 650 and buying a sj or sxr..
I can agree but beyond just that statement maybe this little insight would help you out.
Selling your 650 for 300 to 500 bucks combined with that 250 will get you close to halfway if not more than close to affording a 650 Yamaha Squarenose superjet.
They are going for about $1000 in decent condition and you need to remember that Cash talks as far as maybe waving 800 and a open truck bed to be done and gone with it right in front of the seller. The Squarenose superjets have the same bottom deck as the pre 07 Superjets. This summer I got Tanner thomas hooked up with a pair of Blowsion Destroyer tubbies that he installed in this Sqarenose hull and it only took him an hour.
The big benifit of it all is that you would'nt have to heavily modify the tubbie for the oem exhaust to pass through and or relocate. Further more the 650 Yamaha system will push your body weight around much better in all power points compaired to the 650 kawi. I remember riding my Stock 701 without tubbies and then hoping on my friends 650 square with a prop and it felt and handled almost exactily the same minus the extra grunt and pull in the motor.
My point here is that if you save that extra 300-400 bucks and get ahold a good square nose you can get the ride you are looking for in the surf and not to mention reliability as well. After you ride that hull in the surf for a bit you can then purchase a set of Destroyers when ready and have them on and ready to rock within just a hour or two without the headache of fitting screwing, glassing and or riviting. I swear by the destroyers becauase how easy they install and how well they work. Pricey yes but then again its just about the same amont by the time you add up your extra time and money going into materials making your own pair or another pair "work" on another hull. Just food for thought, I would save your coin, because your close as it is with a few bills in your pocket already. Try that out you will be much more satisfied in the end and be able to ride more. Search Craigslist for yamaha Superjets and if you have any questions about it shoot me a PM and I can help you out or point you in the direction of some people around there i might know that can help you out.