I was just in Trempealeau, WI this week and caught BNSF GP28M 1528 in action working with SD60M 1428. This should make a nice looking model if you bash one.
Dan,
IIRC, EMD did not change the GP's wheelbase until the '40' line came out in 1966(GP38/39/40). The early GP's(GP7/9/18/20/30/28/35) had a 1 ft longer wheelbase than the standard F3/7/9 locomotives.
Jim
Modeling BNSF and Milwaukee Road in SW Wisconsin