The most likely engine would be the big block GMC V-6. Blue Bird put that engine in a LOOOOOOOOOOOOOOOOOOT of buses in the '60's and '70's.
I am trying to remember the sizes. 401 and 478 come to mind but not 432.
If it is the big block GMC V-6, those were not bad engines but they had some odd quirks.
First off, they had an extremely narrow power band. They didn't lug well and once you went out of the top of the power band all you were doing was making more noise and burning up more gas. I never drove one with an automatic. With just a 5-speed you spent a lot of time going slow up hills. A 5-speed with a two speed rear end gave a little more flexibility to stay within the power band.
Second, they had some real balance issues from the factory. They were famous for breaking ears off of the intake manifolds, cracking air compressor/alternator/power steering pump brackets, and shaking the carbs to pieces.
Third, they tended to run hot. It was partly due to the fact they were made out of a LOT of cast iron. And it was partly due to the way in which the combustion chambers were designed in relation to the water jacket. On a long hard pull on a hot day you could get the exhaust manifolds cherry red if you weren't careful.
Fourth, the plugs were in the valley between the intake runners. You had to be very careful when changing plugs that nothing was in the valley that could roll into the spark plug hole. Don't ask me how I know that can happen.
The same basic block was used for the Toroflow diesel engine. The only differences were instead of plugs you had injectors, instead of a ignition distributor you had a distributor type injection pump, and there was no carb on the top of the intake.
By the '80's it was an obsolete motor and as a consequence the parts started to become hard to find. This many years later it could be a real hunt to find some of the engine specific parts like the water pump.
In order to make driveability better and the hunt for parts easier it might be a good idea to invest in a modern electronic ignition and fuel injection system. I know there are kits out there that would fit one of those engines. I have no idea where you would go to find some of the carb parts for one of those engines.