That isn't a GMC engine serial number. It may be a Chevy number but I don't have access to my reference to look it up right now.

Sounds like you got a title for a 1-ton panel truck that had the engine changed and the title changed to match the replacement engine. The door jamb tag shows a 2-ton truck.

"It's just a phase. He'll grow out of it." Mama, 1964

1956 Chevy 1/2-ton 3100
1953 Chevy 6100 "The Yard dog"
1954 GMC Suburban Now with a new proud owner.