Looking to install a Decent Electric Heater, Need advice

[chinaclipper]

I am looking at a Comfort Zone CZ220 or A Newair G73 class electric heater to heat a small, 2 car garage. Estimated square footage ~300 square feet (15x20).
The question I have concerns the connections.
I currently have a 240v plug/receptacle I use for my welder or my air compressor. (one of the other, NEVER both at the same time!) It was professionally wired when we had the kitchen redone to a main panel in the house with 2 120 v 20amp breakers to get the 240 v to the receptacle. 40amps @ 240V, correct?
Can I safely connect the above mentioned heaters (or comparable) using am appropriate 240v plug, or do I NEED to hardwire the heater?

 

[The Cobbler]

2 legs of 20 amp @ 120 gives you 20 amps @ 240 .
5,000 watt heater is at the top end of your ampacity I think. the sparkies will probably chime in with more knowledge