Casting numbers are the best way to identify the motor. People talk about captains bars that are visible on the side of the motor but there is some debate whether they are only on 261s.
Since the 261 is a '54 and later motor the first thing I'd ask is what head do you have? Does it have two bolts in the top holding the valve cover on? If it does its not a 261. A 261 will have the valve cover with four cap screws around the edges. That valve cover doesn't mean its a 261 only that its a '54 or later 235 or 261.
However the truck is a GMC and its not unusual for owners to mis-identify a GMC motor as some Chevy. Does the valve cover say GMC in the middle of the top? If its a GMC motor the size is the first three numbers in the serial number found on a machined pad just behind the distributor. The correct motor would be a 228 but 248, 270 and 302 are all possible.
I have a '54 GMC 1-ton with a 248 and a Hydra-Matic and a '55 1st GMC Suburban that has a non-factory 270 under the hood.
The '53 GMC was among the very first trucks to have the Hydra-Matic which makes this truck special in my mind. The only trucks with the Hydra-Matic before '53 were some delivery models. Those are even more rare.
Nice find!
