Daly has the location for the engine serial number. This site has lots of info on any GM truck, pre '73.My uncle bought a '56 GMC Truck (9600 Series) (3 Ton) and is unsure of the Engine Size.
Needs a tune up & a few parts.
Any help would be appreciated
chevys used buick engines in the hd trucks for v8s the 322 nailhead.Could be a 270 then. But if you look on the crankcase just behind the distributor the info should be there.
GMC's of that era ran their own I6 or Pontiac V8's. I don't think GMC and Chevrolet started using the same engines until the 60's.