Ray
Call me at my office. I own a plumbing and heating company. 406-728-3131
Then I can also ask you some questions about that short block you have
Basically there are some other factors to consider, but 1 gallon of propane = 91,800 BTU's, so just under 1 gallon an hour. A 500 hundred gallon tank can only be filled to 80% which gives you 400 gallons = 400 hours of run time if it ran non-stop. That's really out there though without knowing the actual heat loss of the shop and what teperature you aregoing to keep it at. It also depends on your design temperture at which your heat loss is performed. It should be realistic for the location. Around here we use a design temerature of -10 to -15 @ 70 degrees for a home, shops are usually kept cooler.
I would install a 500 gallon tank.