Shop/Garage Heater suggestions please

We have a furnace that runs off natural gas in our garage. Picked it up on Craigslist for a couple hundererd. We furred our the walls with 2x4 and threw in r13. Then we hung a suspended ceiling with r19 above it. Here in Ohio it has gotten down into the single digits and never had an issue with anything freezing even without the heat on.

When I am going to be working in the shop I fire up the heat about a half hour before and set the thermostat at 60. I have spent many nights working under the dart in a t shirt with windchills below zero outside.