The 348 was the only engine put in the 1959 100 Spartan, there were no other engine options available, so the 409 would be a bolt in replacement motor.
The 1965 Truck 409 V-8 and the 1970 GMC/Chevrolet 478 V-6 both have almost the exact specs...HP (252 vs 254) and Torque (390 vs 430) with the 478 being about 110 pounds heavier.
If it were my truck, I'd keep the 409 as there's really no gain in doing all the work necessary to shoe horn in the 478.
Mike B
