Here's your VIN number breakdown.
S6926F149507
S = School Bus Chassis
6 = 2 ton
9 = 261" wb
2 = School Bus/Flat Face Cowl
6 = Year (1966)
F = Flint
149507 = Unit number
For engine tune up parts, any 1966 Chevy 327 parts (plugs, wires, cap, rotor, condenser and points) will fit, they are all the same.
Looks like a GM 15,000# rear which was standard issue for the 21,000 gvw bus.
The SM-420 4-speed was standard issue transmission with a 13" clutch, but you could have a New Process 4-speed as well as Clark and Spicer 5-speeds or the Powermatic (Allison Automatic).
Mike B
