A couple of clarifications:
First, all gas appliances are rated for input btuh (the volume of gas at sea level, in cfh, assuming roughly 1000 btu per cubic foot of natural gas), and this figure is typically included as some part of the model number designation for the unit. This input figure is what you use to size the gas line, combustion air requirements, etc.
Also included in the unit's specifications is an output figure in btuh since most devices operate at only about 80% or so efficiency. A lot of the heat energy goes right up the flue. The Sooper Heater model 80 is likely to have an output of 65,000 btu or 65 MBH. These are the two numbers you use to select and size your heating system components. Understand which number you need to be addressing.
If the unit is used at elevations above 2000 ft. (exact level varies with manufacturer), the unit's output is then de-rated a certain percentage for every 1000 ft. increase in elevation. Less oxygen at higher altitudes means that less gas gets burned. This is an issue in Denver and other higher altitudes. Also affects fan selections.
As far as heating calculations go, Q=U x A x Delta T
Or in other words, have someone knowlegeable perform a load calculation that you can have some confidence in. While everyone who has posted to this thread is trying to help, anecdotal information and opinions are the wrong way to go. If you have a vendor run a calc, and also make a unit selection, you have a single source of responsibilty, as well as someone who has a vested interest in getting things right. Plus if you use someone local, they will know the climate conditions that are particular to your area.
In reading your posts, I am not clear what indoor temps you are trying to achieve. If your target is 55 degrees, tell that to your vendor and let him/her worry over temperature rise. Some garage operations may require higher temperatures. You can always set the thermostat to any desired set point, if you have the system to deliver the intended results.
Insulation is worth every penny, super insulation not so much. Air leakage/infiltration is equally important, especially if you encounter strong winds. And above all, don't forget that a safe and healthy environment also requires ventilation air.
Knowing that you are proceeding correctly is worth a little extra cost, especially if you end up with the properly sized unit. You will be happier in the end.
Offered only as opinion
.