The way I understand it the 'Advance Design' trucks, 74 thru 50 whenever used all the same cabs on all the trucks 1/2 ton thru 2 ton.. and that the 1/2 ton trucks thru 1 ton trucks used the same fenders, hood, and grill. The 1.5 and 2 ton trucks had bigger fenders and a bigger hood.
The 50's GMC 1 ton near me has what appears to be 1/2 ton and 3/4 ton sized front fenders.
So provided we're talking about an AD truck, if your fenders are too wide to fit on your 1/2 ton, that's probably beacuse they're off a 1.5 ton or larger truck.
