From the looks of things, it appears that there a few things that could be done differently. If you are planning on making a different firebox, then I would patch the hole from the old firebox and re-cut it lower on the tank. Look at the side view of both the smoker and the firebox and visualize the face of a clock on it. On the smoker, I would cut the hole around the 5 o'clock position and the eleven o'clock position on the firebox, then weld those 2 back together. There's nothing wrong with having the firebox in the middle like you have, in fact several people on here have done it like that. Its just more common for them to be on the ends. The firebox may be a tad bit on the small side, but I think if you were to readjust the placement of the firebox, you would see a noticeable improvement. Also, I cant tell, but it looks like you have vent holes for the firebox above the grates? Relocate those to the bottom, a few inches below grate level. Its a great looking rig, and with some changes, you would have one heck of a smoker. Tuning plates as others have suggested, are nothing more than flat pieces of steel set across the bottom to help evenly disperse heat. On something like yours, what I would do, is cut a few pieces of steel and lay them across the entire bottom so they meet the top edge of the inlet where the firebox dumps into the main chamber, (after modifications like suggested above). What this will do is act something like a reverse flow system, will help distribute the heat and the thick steel plates will also absorb and help maintain temps. It will also help the cooking process so to speak by convection. Hope I did not confuse you, but if I did, just message me and Ill try to clarify. Its a learning process and I am in the process right now of making adjustments to my reverse flow. One more thing. It looks like your temp gauge is the same kind that comes standard on all smoker. The kind with the colored areas that read something like warm, hot ideal and so on. Scrap it, and look for a solid reliable guage that reads in increments of degrees and make sure it is placed at grate level as that is where you place the meat. There are several places on ebay that you can get a good reliable therm.
Just a joke... I recently bought a 500 gal propane tank converted to a smoker. I tried to use it over the weekend and couldnt get the temp. at the middle & front over 190 although the gauge was reading 325. My fire box is in the center of the unit on the back with 2- 4inch stacks on the top of the smoker. The fire box is about 30 inch wide by 19 tall with a 22 x 9 opening inside of the box cut into the tank for the heat to go into the smoker. I am trying to see if I need to add a deflector inside the tank to guide the heat towards the front, build a bigger fire box, add more openings for the box to breath, or what. Any suggestions would be great thanks.
