Don't give up the rear tank idea. Anything is possible.
It will work as there are several here that have done it in the past. I am one, but I had no idea what I was doing when I built mine. I guess there is a chance that the calc is based on reverse flow and thats why the numbers are coming up funny.
The numbers I came up with are from using the calc entering 80 gal tank.
An 80 gal tank is 6160 cubic inchs. When you enter the firebox size (square) 19x19x19 you come up with a cucic inchs that is one third the cook area. Roughly. 19 square is slightly more then needed, just a tad.
Then it tells you that 342.95 cubic inchs is needed for the stack. And I believe that 2x4x42 is 336 cubic inchs.
So that should work for the stack, especially since you have it already.
Based on the calc figures for the 80 gal tank, it says you need 20.58 sq inchs of firebox intake.
And then shows 54.87 sq inchs for the firebox to cook area opening. Being said you would need an opening of 9x6 if you made it one rectangle hole.
These are by the calc that we all provided the link for.
BUT!!!!! If I do the math and take 1/3 the tank size for the firebox size, things come in closer to normal.
The firebox is just over 2020 cu inchs.
I am not sure why there is such a difference.
Can anyone please tell me how to find the cubic inchs of an 80 gallon tank? If its 6160 cu inchs then I have the rest of the numbers for the build and they are sounding way better.