There are some folks here who have made the size you are looking into. I'm sure they will be saying hi soon. Frank has plans for that size tank I think it is called the poppa or something like that. I like RF style cookers very well the calc works great for them. Most important have fun and post pics of your progress.
Current Smokers: Backyard RF Offset and Hybrid RF Offset trailer rig with Cowboy cooker and fish fryer, always room for more........
500 gal is a good sized meat making machine. I had Frank make me some plans for my build. The thought of lifting doors that size up even with the counter weights(which I think are ugly) made me cringe so I squared it off with cabinet style doors you could open with one finger which should be highly considered. Reverse flow is the way to go IMO. Look for my first build as it has very detailed pics and steps on how I built that rig.
I been looking at several builds...thinking the 500 gal is abit large. I'm leaning to my original idea of cutting it in half for a more suitable size. If anyone has done this or has any input I'd like to hear it. As a plus I'd have enough tank for two builds.
I suggest you consider the use you plan for. My 275 is about 60" wide and 26" deep with a custom sized trailer. I have 2 shelves inside 12" apart about 52x24". The smoker just fits 30 full racks of ribs lying flat. That was probably a One time annual event. The rest of the time, it's a family of 4 and my daughter hates meat!
Yes I never thought it thru but the thing works so darn good and I have more wood fuel than I can burn in two lifetimes, it's cool. It also gives me the chance to build several other styles and sizes....starting Tuesday by the way.
Just a thought...
Make no mistake, there ain't no powder in this Puff ! And... I'm not really a crazy person but I play one in real life