The 305 v6 is a GMC motor so if your bus has the v6 then I wouldn't think it would be original - unless Chevy schoolbuses were an exception to the rule.
I've personally seen the 305 v6 in an old full-sized logging truck, so I don't think it's totally irrational that it wouldn't be used in a 60 passenger schoolbus. The bus might be seriously slow when loaded down with students but it would eventually get them to school.
I would think that the 327 (smallblock) or 366 (big block) v8 would be the standard gas engine for a bus of that vintage. Plenty of power and torque to get a loaded bus moving easily.
-ftyler
