I would say that a 261, out of a '54 model would definitely be a easy swap. I pulled my '53,& '54 Chevy COE's next to each other, earlier this morning,('53 has a 235, '54 a 261) & I dont see any visable differences. Don't know about the later, 261's, all of the '55, & up trucks I've had were V-8 powered.
