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How big is your cabinet? And what do you intend to smoke?
Typical Fridge size conversions use a 14,000 BTU burner or thereabouts. The problem with going too big is that getting lower temperatures is near impossible because the burner won't stay lit at such low gas flow. My bottom end is 150 degrees in my cabinet using a C-burner: http://www.gassmoker.com/c-burner.htm so when I want to smoke fish low and slow I need a small flame from a bunson burner to get the 120 degrees I need.
58,000 is in line with the big offset smokers I've seen though...