McIntosh manual recommends 12 inch of air space above the amp(tube) for adequate ventilation. My rack only allows about 8 inch of space above it. Does it sound like a problem that I should address with a cooling fan perphaps?
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.