It all started with a text "hey man, got a free smoker for you. some assembly required"
45 minutes later, this was sitting in my driveway and the gears were turning.
As luck would have it, not a week before a forum friend called asking if I wanted some metal carts. Having just bought a welder, I said sure, figuring at the very least I could chop them up and put them back together. But I got lucky with one cart that had a removable top, and the 30 gallon air tank fit almost perfectly.
First things first, I needed a door and to find out why the tank sounded like it was full of gravel. So I turned to CAD (cardboard assisted design)
I made it look about how I wanted, traced it out with a paint marker, and got the angle grinder and sawzall ready. My first cuts were the top corners. I went in about 8 inches on each side, and tacked my weld on hinges into place thinking it would help prevent any warping or bending.
Oh, there's that gravel noise. About 2 gallons of rust.
I decided now to do a mockup with the firetank, and realized my pretty autocad drawings didn't translate into 3D very well. I'm resourceful though and eventually found a work around.
Just like with the cook chamber, I cut the top, and welded on hinges before cutting the rest of the door out. It worked out for me, so I recommend it even if it isn't common practice. And my first big mistake, that I still haven't fixed.
the clamps moved as I was welding and I didn't notice until I was done, so the bottom "bracket/gasket thing" is VERY crooked, but still almost seals. 
I'm still not sold on this either, I welded the gasket piece for the back on the inside instead of screwing around with the hinges. It didn't leak in my test fire, at least not very visibly, so I think it worked alright. I was then given some great advice for a contour guage
but when I made it, it didn't work very well at all, so I had to fudge it. 
This was my solution to joining the tanks together. Actually worked out pretty well. Turns out a fire extinguisher is the perfect size opening, according to the feldon calculator, and I have a bunch of dead ones laying around. 
Fast forward to everything being together. The wall of the tank was so thin that just the pressure of my welding wire feed was punching holes through the rust, so I friction fit it, and put on 2 coats of JB Weld extreme heat, that claims a 2200F tolerance, to seal the gap. 
That led to my next major screw up. I didn't own a plasma cutter until 4 days AFTER i tried to cut this hole out with a grinder. The original round hole came out good, but making the adjustments to fit the chimney in left me GAPING holes all the way around that I welded patched on, then covered heavily in Steel Stick. 
Now here she is, all put together(mostly) and ready to rock. Added the big pneumatic wheels to get it around on my gravel and swampy yard, 1/2" round bar with spring handles that do a great job keeping it cool, and the red brackets to lift it higher for the latch I haven't installed yet.
It has now been test fired and seasoned. I've maintained 500F in the cook chamber for an hour, so I'm calling it good.
I still have 1/8" x 1" fireplace gasket to put around the door, the latch to put on, a mesh firebox insert to make, something for the gaps on the firebox door, and paint. Which means at 90% done, it's pretty much finished.
This was originally supposed to be a reverse flow smoker. I couldn't make a good pattern to fit the round end of the tank with cardboard, so I haven't tried at all with metal, but it may not need it.
As a bonus, I have 150lbs of insulating cement just sitting here looking for a home, and am considering putting a 1" thick layer across the bottom of the cook chamber to help hold heat when the door opens and maybe even out the heat distribution, but I haven't seen much of any information about combining a smoker with insulating cement.
She's definitely one of a kind. 
I will say that it looks a little tipsy with the wheels being so close together.
It's going to live next to the blue table. I might move the blue table first and just put it next to the New Braunfels I'm adding a rotisserie to, it's still up in the air, but the end of the shop seems to be a good out of the way yet easy to get to place.
and 
The door on the firebox now has even bigger gaps. I must have warped the door when I welded the flaps on. 
but the gaskets on the cook chamber make a better seal. 