This is some bad memories since I now live in Texas, but grew up in CT where I did a lot of exactly what your trying to do, only it was a '68 144.
One time, I needed to replace the windshield wiper mechanism in the worst cold snap In January. It was -5 degrees outside, and the garage was between +10 and +15 degrees, a rickety old wooden building with cracks which let wind and snow drift inside.
I did the job at night, having the same situation you have. I finished about 4 am when the temps were still plumititng. Since it was at night, I needed a trouble light, and used (very sparingly used!) one of those quartz work lamps that can light up a backyard, 600 watts and some good heat also. I pulled out the seats so I could work inside the car while the doors were closed. I pre-heated the car with the radiant heat for ~10 min while I warmed up inside the house. I pointed the lamp under the dash so the parts were the only things getting warmed. While I was working, I used a conventional work light since it was so hot the quartz lamp would give me a sunburn.
When I was done, I drove to Vermont for a day of skiing at Killington.

The thermal gloves at Home Depot are great. Get the ones with rubber nubs and thinsulate, great for handling tools, they are only about $4.00
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'89 245 sportwagon, destroyed by hit & run driver, RIP. '04 V70 2.5 T Sportwagon, 12k mi and '91 245 5-speed, 209k mi, replaced the '89