My shop is 60 x 80 x 19. I have two heatstar by enerco 150000 btu tubes.  They are 60' and on separate thermostats.

In 2013 they cost me $5700. installed.  My local CPS (lp dealer) had them engineered according to building and they installed.

NO problems and use very little lp and keep floor nice and dry! Keep mine around 58 degrees until grandkids come and then turn up a bit.

Also only about 15-20 minutes for them to rewarm after door opening.

Sounds like one would be perfect for you! 

gopher