The engine is obviously much newer than the bus, since 261's didn't begin production until 1954, and the 1959 casting code on your engine confirms that. The engine that was originally installed would probably have been a 235 with spray oiling of the connecting rods and Babbit bearing rods. The 261 is a definite upgrade, power and reliability-wise. 235's that were built from 1941 to 53 were basically upsized 216's and were only used in medium-duty and larger trucks. Passenger cars didn't get a 235 until 1950, and then only ones with Powerglide automatic transmissions.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!