Should be, but FX375F2356F looks like it has it has a few extra or misinterpreted digits in it. I don't think the X would be found on the original tag on the truck, and the second F should have been a P (from the Pontiac Michigan plant) or a C (from the Oakland Ca plant). You can find more info about the serial numbers and tags here OldGMCtrucks and the labels here OldGMCtrucks
I would expect your chassis serial number to be something like F375P2356F.
More info about your 375's running gear can be found in the front of the Master Parts Book. Here is an excerpt and a drawing from the same book.
'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.
