Our booth for painting trailers has in-floor heating in it. That is the cat's meow. Plus when you are rolling around on a creeper and your shirt slides up you don't get a surprise when you slip of the creeper. Just warm concrete
In your shop, I wouldn't run the heater even if you were taking your air from the outside... just too scary. Seriously, I don't think you will have any problems if you turn your heater on 3-4 hours ahead of time. Your metal will be warm through and through and your air temp will be fine. It shouldn't cool off past 60 that quick. I am not sure what the proper range is for painting but maybe get your inside heat up to that point before you shoot. You might sweat at first but it will take longer to get to 60 degrees.