275 gallon fuel oil tank smoker

To get the volume of your oval tank, try the oval tank calculator on the Calculator Soup site or just take your gallons (275 x 231= 63,525 cubic inches):

http://www.calculatorsoup.com/calculators/construction/tank.php

Go from there with the rest of the calcs:

63,525 x .33 = 20,936 cubic inches for your firebox

63,525 x .004 = 254 square inches for:

                                                              - FB/CC opening

                                                              - Area under the RF plate

                                                              - Area required at the end of the RF plate

Etc.....

Hope this helps.  FYI, I posted all my calcs on my ongoing RF thread (200 gal Oil tank - 55 gal drum firebox) as an example.