Hi Space,

Personally, I wouldn't increase the prop/gearbox shaft at all. In fact I'd do the opposite in order to keep the prop as close to the top bearing as possible. In theory, the longer the shaft, the more preasure you'd put on the bearings, especially if the shaft was not perfectly straight.
Much easier to remake the frame with a pair of V shapes, in order to lower cg. CF rods glue great with 24 hour epoxy.

PS: How much you selling your PG-03s for ?

Cheers ADI