This is the one I built for the son,
Used a hot water tank for the fire box. It also is in 4 trailer Jacks so it can stand on cinder blocks to use or so he can use the trailer for other things.
I will say that make you cut so the side 1/4 of the tanks opens with the top of the lid over the grill smoker area. We followed the advice of a so called expert and will be bringing it back to weld a patch over the cut out door and cut it so the whole 1/4 of the tank will open. It will be a lot easier to seal up the cut lines straight rather than on the curve.
As to cleaning it out, drain the tank completely unless you know for sure what was in it and when. This tank had about 2 gallons of oil not water as my son was told.
I like Dawn industrial/commercial cleaner cuts oil, grease very good. I get it at Gordon's food service store. I use my power washer pour a lot of the degreaser in. Fill by using the power washer to spray in the bung openings all directions. Let sit for a day or two. With all the air out you can cut with it filled. Or I drain and rinse out with the power washer, Depending on how you are cutting I used the cut off wheels and a die grinder or the air cut off tool with the wheels.
Son didn't feel that he couldn't do a good job with the plasma torch. It was his first time using one.
This is it loaded up and heading out to his smoke with it 100pounds of pork shoulder for a shindig at a veterans home.
You can see the opening for the grill area that we are planning to change.
After it was opened, we sprayed vetabale oil in all the seams and over the flat surfaces. and used a turbo weed burner torch to burn out the residue. Did that twice and then the son fired it up and did a wood burn in it. Then he did a test smoker and not a hint or smell.
They eat everything at the cook out, and sopped the drippings up with bread out of the pan's. After the pork was gone.