I suppose that if a dealer or fleet buyer wanted the 283, and ordered enough of them, GM might have built them on a COPO basis. I do know that they did build trucks in some of the same plants as cars, so a 283 being slipped over to the truck side isn't inconceivable. But none of the factory liturature gives any reference to the availability of a 283 in the 1/2-1 1/2 ton truck.
KillerKnight, it really isn't too odd that a truck would have the 6 rather than an 8, trucks of that time relied on low end torque instead of high rpm horsepower. Low end torque is something that an inline 6 has in great abundance compared to the v-8. Also, given the gearing that a truck has, the 6 was all that you really needed.
On the other hand, wouldn't it be really cool to find one with the Feully Corvette 283, maybe in a Cameo?