If you have at least two circuits of 110V power, a pair of 1500 W electric heaters may do it. Leave one on overnight heating the block and put both on for several hours before painting. Goes without saying, keep them away from the paint or turn them off when painting.

Make a small booth out of wood and plastic or PVC frame and plastic maybe 8 foot square put a small electric heater in there for a while and you will be all set of you use one of those oil filled electric heaters there is no worries of fire leaving it on while you paint.

Go to your local Home Depot and buy this Quartz infrared heater http://www.thethingsiwant.com/item/1354218/ It's $30 bucks and works great for spot heating in a cold garage. Put it about a foot away from the block for 30 minutes then switch it to the other side for 30 minutes. Block should then be nice and toasty. Spray away, let the paint flash and put the heater back on. This time keep it a little farther away, rotate to the other side, keep this up until the paint is dry.