I think they're probably okay outside if the temps are warm enough. The best way to know is by observing them. If they stand in a huddle making a lot of noise, then they are cold. If they move around freely and/or sleep quietly in a pile, they are fine. If they are lying down with their legs stretched out and panting, they are hot. Check on them frequently for the first day or two, because they may seem fine for a while but then gradually become chilled or overheated.
If your temps are reaching the 90s & 100s, the biggest thing you need to look out for is *over* heating. Make sure they have plenty of cool water--a very shallow pan that they can get into and out of easy but that they can't possibly drown in will help--and make sure they have access to shade all day long.
60+ degrees is probably fine for a short while, but they'll want some supplemental heat for sleeping under. Can you run an extension cord outside and fix the heat lamp where they can use it?