I ran two 400 watt 10K AB's on a 75 for quite a while and didn't have too many heat issues but I had a good size clip on fan from WalMart running over the water. Lot's of evaporation though (about a gallon a day) so be ready for that, I'd say my water never got above 80 to 82 which was a little hight but not terrible. I did it for the same reason so I could upgrade later but now I'm back to 250s on my 140.
Just keep an eye on the temps and your evaporation rate.