With those high output propane you heaters really need to run a 100lb tank. I mean you can run a 20 pound tank, so long as you keep air blowing over it (THE TANK) to keep it from freezing.
I usually run mine at 200,000 BTU for a few minutes at a time, then shut it off. After that i'll periodically run @ 100,000btu to keep the temps up.
No clue how harmful the emissions can be ( almost everyone says it's safe but i've yet to hear an expert opinion). That said I constantly keep fresh air Cycled in by cracking the garage door a bit.