Have you done any of the standard horizontal/offset mods? Replace thermometer, stack extension, charcoal basket, baffle, tuning plates? Not so sure you need to seal it up but I suppose you can if you want.
As far as another fuel, I converted mine to propane but it certainly isn't requried. I did it for convenience and because I can't get decent lump in my area.
Give us a little more info and we can help you out a lot more.