I use a 24x48 inch flat pan on a 275 gal. oil tank arch, it has 1 inch angle iron around the perimeter to support the pan; before that I had built an arch from a very old water heater and used a 16x24 inch flat pan. I've since modified the oil tank arch to incorporate the 16x24 flat pan; then I added a copper coil preheater around the chimney stack, and after that I added Air Under Fire. All were great improvements, but without question the most noticeable improvement is the AUF, so plan to incorporate that from the get-go. I boil with this rig on weekends only; collecting all the sweet, the 16x24 pan does double duty as I use it as the finish pan on an electric range I modified into a six burner unit. I boil about 500 to 750 gallons of sap per season. Insulate the hell out of the oil tank to minimize heat loss/maximize heat to the pan.....ceramic blanket, archboard, firebrick, etc. Did I mention AUF? I really like the set up I have and I think you will really like the step up in performance from the turkey fryer, which I did and it took me only one season to get out the tools and start fabricating. Good Luck and post some pics.
2010 - 12 taps, turkey fryer, 4 quarts
2011 - 24 taps, homemade arch from old water tank, 16"x24" flat pan, 16+ quarts
2012 - 9 taps, 3 pints, what a season
2013 - 60 taps, homemade oil tank arch with 2'x4' flat pan, 16"x24" finishing pan on electric range, 55 quarts
2014 - 80 taps, homemade oil tank arch with 2'x4' flat pan, 16"x24" finishing pan on electric range, 40 quarts
2015 - 100 taps, 15 gallons
2016 - 115 taps, 13.5 gallons
2017 - 120 taps, 13 gallons
2018 - 130 taps, 11 gallons