The link above is the calculator with your tank dimensions, you will need to cut the tank 11 inches up from the bottom for the top of your firebox and that will be the height of your RF plate. At the end of the RF plate, you will need 250 sq-in of area where the air turns and goes up to the cooking grates. I plugged in the sizes based on the 100% firebox; however, you can make it as big as you want. It will need to be approximately 33 inches wide since that is how wide the cut will be if you make it 11" above the bottom of the tank. If you use a 7" pipe, you can get by with under 30 inches of exhaust stack height.
Hope this helps and keep us posted on your progress!