I don't know about 235 vs 261 differences for all years ('54-'62) but here are differences between Thriftmaster 235 and Jobmaster 261 engines (for 5000 and 6000 series trucks) in 1954:
- camshaft
- carburetor (for 6000 series is different)
- PCV on 6000 series
- block (obviously), head, and head gasket (6 more water holes on the 261)
- pistons, piston pins, connecting rods
- 235 Loadmaster valves in the 261
- valve springs
- valve rocker arms. push rods, rocker shaft (wall thickness)
- valve tappet (lifters)
- fan (increased pitch on the 261)
- water pump and hoses are 1/4" larger inside diameter on 261
- thermostat and housing
- engine (distributor ignition) timing (flywheel marks)
Some of the differences above refer to parts that might be interchangeable but they are not identical. Most of the valve train parts, for example, are "unique" to the 261 but the valve train parts might be interchangeable (same dimensions) between 235 and 261. The beefed-up valve train was designed to handle the higher lift of the 261 cam.
The Loadmaster 235 was optional on 4000 series trucks and it had a few of the features of the 261 (that the Thriftmaster did not have).
This information is from the 1954 Chevrolet Truck Engineering Features manual.
The '58-'62 261 has a full-flow (not bypass) oil filtration option. The 235 never had a full-flow oil filtration option.
Tim
