@Perry, I was hoping you could benefit from what I would call a mistake in my garage installation...
At the time I insulated my garage there was no code requirement for drywall in the garage if it was detached. I should have sheathed my walls and ceiling with 3/8 or 1/2 inch plywood instead of drywall. It would have saved hundreds of little annoyances that made installation of cupboards, hangers, and even electrical more difficult in the 'industrial' setting of my garage.
I installed my own ceiling furnace. To do so I installed 1/2" plywood over top of the existing drywall, which worked well. I used the existing gas line, and extended it from the floor to the ceiling...
After having two bad gas installation experiences in the past (water under the bridge) I install all my own gas appliances these days - main furnace, garage furnace and water heaters.
So for your own protection, after the install guy pronounces it 'done' and before you turn it on, go to every joint with a spray bottle of water with a teaspoon of liquid dish detergent, and spray every joint... also meticulously check ever vent joint. I made a mistake with my flue on installing my garage heater, discovered by Janger - but it was quickly and permanently corrected. Anyone can make a mistake! even professional installers...