Air space for McIntosh MC-275

I would say monitor the shelf above and see if it is getting too hot. If not you probably have adequate ventilation.
My MC352 is in a smaller space than recommended and it never gets too hot to touch.

I agree with Theo. The shelf above may get hot but will be okay depending on what material it's made out of. The amp itself will not overheat.

The shelf above it is not my main concern, b.c. the amp does not run all that hot. Amp overheating and tubes longevity is what concerns me more.

With all sides open and still having 8 inches above, I don't think you will have any problem at all with amp overheating or tube longevity, and do not need a fan. If you wanted to be extra careful, you could run the amp with the cage off, but again, I don't think that is necessary.

I agree with Rgurney that setup sounds fine. Obviously check from time to time for overheating on the shelf above, but ventilation sounds more than adequate. The McIntosh recommendation is probably aimed more at less knowledgable consumers who may be putting their new amp in a closed cabinet.