Hmm...getting the doors to seal up tight after making cut-outs could be a challenge. The heat from standard cutting methods can warp the arc of the steel if you get in too big of a hurry. Some heating and re-shaping may be needed to get the proper fit.
BTW, looks to be a vehicle propane tank...I'd plug all the side ports and fill 'er up with water and keep the water running into it before you make a cut. A plasma cutter would be the safest route to go, as you wouldn't be injecting oxygen into the cut from a torch head. Man, I hate the thought of cutting into a fuel cylinder, myself.
Be careful, and cover all your bases before you make a cut.
Judging by the lower access door configuration, I'm assuming charcoal/lump fired, possibly with a water pan?
Have you considered a reverse flow horizontal? You'd need a side fire box, but...if you have access to hardwoods for fuel, they make great stick burners and cook evenly...