"The V8 conversion only" clause probably works because the V8 engine is shorter front to back.
They aren't cheap, but searching for "CC46167 radiator" finds a part that is supposedly GMC specific. Cold Case (the manufacturer) has them as SKU GMT559A.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12
'52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
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