I am so jealous that I hate you!
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I have been hoping to come across something like this on a demo job or the likes. Nice looking box! Great score and even greater potential!
I would say to go with electric heating elements from any number of sources (internal or external mounted) and then add an external smoke pistol type smoke generator. Is there any heating capacity already with or built into the unit? (You really don't need high temps)
I say electric for several reasons
-relativily easy to find parts
-realitivly easy to adjust temps
-no open flames to worry about
-no cumbustion gases
-no need to allow for air supply
-no fuel to run out of
Yaddy yaddy yadda, It will ultimatly be you choice in the end, but I am biased.
I would also lean towards an out of the smoke chamber heating and smoke generation set up. Mostly in this case because you have such a nice smoke chamber and I wouldn't want to waste an inch of the available space. Another reason could be that you could easily get another already made firebox and attach it to the side. Another reason could be that fire or smoke tending can happen out side of the smoking chamber so that you are not opening the big doors and releasing heat and you are not tending a wood fire in the smoke chamber and stirring ashes. Another reason could be that you would not have to worry about drippings on or into your smoke or heat source.
I don't know. Maybe if it were me, I would get an electric oven element that was shaped to fit the smoke chamber, the controls for that element and use them under a water pan or a double layered baffle/heat sheild w/drip pan. I still think that I would atleast keep the smoke generation out side of the chamber using a smoke pistol type set up or iron skillet on a hot plate with the smoke piped to the chamber.
Tell us what you have to work with like materials, skills, money, tools and what you think your perferences are. Do you want wood fuel, charcoal, LP, NG or electric? Everyone here will look at that big beautiful box and have their own dream about what to do with it. And most of them will see that thing just slam packed full of meat with just a little bit of Thin Blue Smoke waifting out of the top.
Keep us posted with lots of pics, this is what we live for!