Was just doing some thinking about what would be necessary to convert the GZ250 to a 350 Using DR350 parts. This is of course assuming the parts would physically fit. You would definitely need the following from the DR350 engine:
Piston
Cylinder sleeve
Crank shaft
you might need the following from the 350 engine:
cam chain
cylinder head.
The way I would envision this is that you'd have to swap the crank shaft to add the extra 10mm stroke. the 350 sleeve would be necessary to accommodate the increased stroke and then obviously you'd need the 350's piston. I'll assume the connecting rods are the same. I would think you should be able to use the 250's cylinder head. This would make it easier to not have to do any custom exhaust work and worry about the carb fitting. I also would think the cam chain could be reused from the 250, but you may have to use the 350's cam chain if the 250's chain doesn't fit due to increased stroke distance.
This would seem to be cost prohibitive and time consuming. A more economical approach would only get you a 300cc engine but would have more likelihood of success. This would be to have a machine shop bore and resleeve the GZ250's cylinder jug to accommodate the DR350's piston. The only problem would be whether or not there is enough material in the GZ250's cylinder jug to fit the larger piston sleeve.
I hope this give you some food for thought. Motorhedfred, I see you're in Ohio too. Depending on how far away you are if you do decide to attempt this, I'd be willing to offer a hand.
Give these guys a call maybe they could come up with something.
http://kustom-kraft.com/SUZUKIKITS.html