No problem, it just didn't compute there for a minute. Click on this link.
It shows you need a firebox to cook chamber opening of about 216 sq-in; however, it is recommended to use 1.5 times that value to assure it will flow well. Therefore 216 x 1.5 = 324 sq-in needs to be the size of the cutout and the area under the cook chamber plate. To get that number, you need to cut the tank opening up about 12" from the bottom when you look at the tank from the end. If you put your cooking grates on the 9 o'clock or at the tank midpoint (18" up), there will be 6" of clearance from the grate to the cooking plate. You need the same 324 sq-in at the opposite end of the cooking plate from the firebox for the air to make the turn towards the cooking grates. The calculator link shows the exhaust pipe size needed and you can play with the inlet dimensions to make the vents work like you want, but I plugged in 6" x 3" as an example and you would need 4 and a half of them, so go with five, three low at fire grate level and two in line with the cutout to the cook chamber. Play with the firebox intake box so you get them like you want. There are lots of really nice examples of both round and rectangle within some of the builds. Hope this helps and keep us posted on your progress. Nice score on the tank!