Hopefully some of this will point you in the right direction, at least... IHC built about seven V-8 gassers in 1964. I notice one of these has what appears to be an oil-filler tube mounted on the front of the engine, I wonder if this is unique to one of them?
Here is a screenshot from the Wikipedia on IHC engines, it has a specific section on gas V8s... nothing about identification, however.
International V8 Gassers.jpg
This link indicates how to identify 304 and 345 CID variants in the Scout
... I'm kinda thinking the engine ID pad noted in the article may be universal to all of IHC's gassers? Like perhaps that area on your engines may read V-392, V-404, or V-446? FWIW, I'm thinking most of these buses got 345s... I believe the bigger engines were reserved for the larger trucks. But if you're lucky enough to find you have a 392 or larger, you may have a very desirable engine, depending on if anyone is looking for one.
Hope that helps!