I think I solved the problem. I searched for "wheel adapters" and none for 105mm. But I noticed a "4/4 to 4/110" adapter used for golf carts. Turns out that a 4/4 bolt pattern is a 4" spacing, or 101.6mm. So I went back and PRECISELY measured the bolt pattern on both wheels and got 102mm or 4" exactly. Carter is putting 4/4 golf cart hubs on the front!
A set of 4/4 to 4/110 adapters is about $100. Here is one web site that sells them:
http://www.kgmotorcycletires.com/atv_wheel_spacers.htm
And another:
http://www.motoxmall.com/atv-wheel-a...0-adapter.html
The tech support guys at Carter did not even know this... I am gonna call them back tomorrow and ask directly.
As for the ITP SS wheels. It looks like I will get the set of adapters and keep the rims. ITP does make golf cart rims that should fit the front of the GTR.
http://www.itpgolf.com/products.html But I need a 14x6, and they only make the SS212 in a 12x7 rim. They do make 14x6 rims in other designs. This should resolve some headaches for those out there wanting to put custom rims on.
So all you GTR 250 owners:
4x137 on the back, ITP makes them for ATVs.
4x110 on the front if you are in luck and got this type of hub.
OR if you got 4/4 hubs:
4x110 on the front with a 4/4 to 4/110 adapter OR just use 4x4 go kart rims, which ITP makes also in designs that match the ATV rims.