Could be anything in a 60 year old truck, but more than likely it's a GMC engine. GMC and Chevy were quite different back in the day. If you could get the stamped numbers from the engine, which should be near the starter, we could narrow it down for you.
Welcome to the 'Bolt!
Scott

Scott Ward

2 1948 1.5-ton Loadmasters
The red one and The snow pusher
1 1950 3100
1 1955.1 Chevy 6500
1 1954 Chevy 6500 2-Ton
1 1955 1st Series COE 5700
1 1963 K20 (454)
1 1964 C10 (350)
1 1951 1.5-ton Dump Truck
1953 and a 1956 Ford F800

Raising a teenager is like trying to nail Jello to a tree!