I repeat: the Cadillac 472 and 500 V8 are tortally unsuited
for this type of use.
The 500 Caddy has an obscure little brother, the 472.
472's are more common...they ran in all 1968-69 Cads (RWD and Eldos) and in 1970-74 RWD cars. 500's ran 1970-74 Eldorados and all 1975-76 Cads except Sevilles. 1970-up 472's used the same blocks as the 500's. The big change was in about 1972: the switch to open-chamber heads.
I have seen a late 70s-early 80s Fleetwood with a 425 in it, but it may have been the Olds motor
1977-79 Cads (de Ville, Fleetwood, Service Chassis, Eldo) had Cad 425's, the only Olds was the diesel. The 368 (V-8-6-4) was externally very similar to the 425, showed up in, IIRC, 1980. Later Cads had the Olds 307.
The Buick 430 ran alongside the 400 back to 1968 or earlier...it got less notice since it was never installed in anything smaller than a land barge. The 455 replaced it in 1970, also alongside the 400. The Buick 455 ran to about 1976, the 350 a couple years later. The 455 is a stroked 430...same block. A 430 block with a 455 crank & rods is a 455 and vice versa.
None are especially well suited for powerring school bus.