There is no correct orientation for the fan, no matter if it pushes or pulls, it will be ineffective all the same. The fan is a bolted on kludge to fix Job's idiot design philosophy. The logic board isn't what you have to worry about, it's the analog/power board where all of the heat is generated.
Steve Jobs demanded that the original 1984 Macintosh have no vents, holes or fans of any kind "because MY PRECIOUS". The final design had tiny slits for convection, but that was grossly inadequate. As a result, predictably the machine hideously overheated from having a smoking hot vacuum tube in a hot box. High failure rates and warranty repairs galore ensued. Jobs never learns from his mistake, this wasn't the first time, nor the last that he would create a hot box oven that baked itself to death. The Apple /// from 1980 had even worse problems with heat because it had literally zero vents. The 1998 iMac G3 was another with just convection cooling, it also gets smoking hot.
It wasn't until the Mac SE in 1987 did the compact Mac eventually get a fan. It and the SE/30 were the *best* cooled out of the lot, and they still had overheating issues on the analog board and power supply.
The Classic has the fan in a weird location because of the combo analog board + power supply, CRT and memory expansion board prevent the fan from being installed on the rear.