From my tours of the factory, my take is that Gulfstream builds most of the fuselage in house, in Savannah.
I think they want to have a differentiated product, which is an upgrade to the 450/550. The 450 is an old dog now, and while the 550 is still a fantastic machine, it seems reasonable to make two versions as long as you're building a new bird. The 650 needs to be the big daddy, and be different from the pack, so it gets the wide body treatment.
Although the Bombardier lovers look at the wide cabin as being a fantastic advantage, most G owners feel that the old-school width is fine. What exactly do you need the 5 extra inches of width in the cabin isle for?
FR