The only concern I'd have with putting the heater in and keeping it at 79 is once the lights come on the temp will most likely spike higher than the 81 he's getting now, basically start higher end higher. Possibly putting a small fan on the same timer you have your halides set to might help.

My old tank used to have a range around the same and I never had any problems with it, maybe some specific species that are very temp sensitive may have a problem though.