I ran my old Model 750 in an open rack (just sides, no obstruction front or back) with just over 1" clear above it. That's less than I recommend (call it a case of "do as I say, not as I do" ), but the air space above the 750 never got much above 100F even in the summer. I've got about a foot above my 7500 right now, but it's also in a closed cabinet. Ideally, I'd try to have 3" or maybe 4" clear above it.

Depending on how deep your rack is, it may end up being open at the back to accommodate the full depth of the amp and the cable connections back there. The backs of 7500 and 990 are both just above flush with the back of my cabinet, with openings cut out for both (making wiring much more convenient).