I have a custom build equipment rack and if I go with the 990 I will have about 1/16 of an inch on top with about 1 1/2 on each side. How much air space do they recommend for this unit, guessing thats not enough. Thanks

It's probably not enough, however, my 990 has not got hot at all. I had War of the Worlds on pretty loud and felt the 990 and the 755 amps afterwords and neither were even warm. I would try to leave at least a couple inches on top. Is the front/back of the rack open?

Yes, the front and back are open.

I always try to leave at least an inch or two above any piece of equipment. However, the 990 doesn't seem to generate much heat at all. I wouldn't worry about it.

Cheers,