Whatever type of heating appliance you heat it with, sizing it correctly for the load keeps it from short-cycling, and insulating the space makes a large difference in load, allowing for a smaller (usually cheaper) heating unit, and lowers the annual fuel use. Heating an uninsulated space in Maine to even 50 F is an expensive proposition.
So, run some sort of reasonable load calculation don't just guess and end up with something 5-10x oversized. Run the load calc on both the current uninsulated case, and at R10 wall insulation.
Note in central ME where the design temps are in double digits the basement/garage can be a large fraction of the total heat load if uninsulated, even if the basement/garage is only kept at 50F:
A 2x6/R19 wall at an 80F temperature difference (70F indoors, -10F outdoors) loses about 5 BTU/hr for every square foot of wall.
An 8" CMU wall at a 60F temperature difference (50F indoors, -10F outdoors) loses about 30 BTU/hr for every square foot of above-grade wall.
That's six times of the heat loss per square foot of wall you have upstairs when it's -10F outside, so it doesn't take much above grade basement/garage wall to add up to the entire wall losses of the insulated walls upstairs(!).
But an 8" CMU wall with R10 continuous insulation at the same 60F temperature difference (50F in, -10F out) only loses ~5 BTU/hr for every square foot of above grade-wall, comparable to the heat loss per square foot of the 2x6/ R19 wall at an 80F difference.
The benefits of insulated foundation walls continue below grade, but the exterior side of the foundation doesn't drop to -10F when outdoor temps do. But even at the frost depth level (more than 3' below grade most years in most of Maine) and a 50F indoor temp, 30F exterior side dirt, the uninsulated wall is losing ~10 BTU/hr per square foot, while an CMU + R10 wall is losing only about 1.7 BTU/hr per square foot.
That's heat you're paying for, whether you're thinking about it or not, and whether 50F feels warm to you or not.
If using an 80% combustion efficiency wall-thingy to heat that space it'll use 15% more propane to produce that heat than using than condensing boiler would, but that efficiency hit is small potatoes compared to insulated vs. uninsulated.