I think it would be better to find a way to heat the garage.
You'll need really powerful heaters or and multiple ones to heat those temps. Which seems like a big waste to me.
Not sure 1k would work for a 180 gal in those temps. At my parents house, temps vary between 50- 60 F.
I've have two 300 watts heaters in my acrylic 150 gal at 4 per gal and it''ll heat up to around 70 F.
When I have third 200 watt heater in there which I am currently using my glass 55 gal and the room temps go to up 60F...then it heats fine.
So I guess in conclusion...five watts per gal in an acrylic tank works in 60 F. I think you'll need more.