also take a look at the topkick chassis with the 7.8 isuzu motor, they are reasonalble and seem to be reliable, a little bigger than what you wanted but I just saw a 4 year old crew cab one for 22k with low miles with out a bed and it had a long chassis...
my mid 90's 3500hd 14k gvw has 84 cab to axle and had a 13ft utility box on it when i bought it... it can be done...