gfs

I have an old 1950's fridge that I want to make a GFS out of. Does anyone know of a site that has a tool calculator to help with fire box dimensions, exaust, air inlet dimensions, etc?or is it just whatever looks right?

I'd really love to build a gravity feed out of an old 1950's fridge I've come across. Now, is this do-able, and if so, should the fire box still be 1/3 the area of the fridge? I was also wondering about the exhaust. Am I better off cutting a hole in the back or sticking a pipe down through the...