The problem with heat stress is.. it is very specific not only to the individual rabbit, but also the genetics of the rabbit "line"... a general rule is 90 degrees F for buck sterility. - I have seen individual rabbits get heat stress at 85 when pregnant and close to the due date.
You will have to watch your rabbits and look for the signs that show they are becoming uncomfortable. If the rabbits get too hot it takes a lot out of them, some rabbits are never as healthy as they were, after getting too hot. I have always kept a few 2 liter plastic bottles of water frozen in the freezer for "emergencies". I hold it against their stomach if they are "in trouble", or just let them lay against it if they are looking a little uncomfortable. In rare cases I have filled a sink with 80 degree water and submerged them in it. [except their face] -- if they survive, .. I use a blow dryer on "warm" [not hot] to dry them again. I have used ice cubes to rub against their ears if they look like they are "about to get in trouble"... there are some pictures of heat stress here.. but also readily found on "the net" -- I suggest you formulate a plan ,to keep this from ever needing to be treated. I have used a closed building with swamp coolers when "out west" . and the same with air conditioners when in the humid "East".
I have used big "circulation fans" , to keep air moving fast, over the cages.....
In Florida...I had wire cages, inside an open front shed , under big trees in 100% shade, and recycled carpet over the entire shed [including most of the open front], - with a sprinkler on top ,that came on at 80 degrees to wet the carpet. I had no heat losses ,..except a time or two when the power was out, and i didn't know because I was away working.