Lance,
I calculated 45000 BTU/hr assuming the following:
16x25 garage (400 sq ft)
8 foot ceiling
walls are brick on four sides
roof is corrugated asbestos
floor is concrete
ambient temp of 10 deg F (and you want it to get to 60)
20% heat lost through ventilation
50000 BTU/hr sounds about right, especially if you are losing more than that assumed 20% through ventilation (leaks).