200 gal tanks would be a mistake as the size would have a very negative effect on drag.
~115-135 would make more sense as the big 165 US drop tanks were generally only used
for ferry purposes, and 200 gal tanks would be big.
So a couple of 115 gallon tanks (is that US or Imperial by the way ?) would give it a range of approximately 2,200 miles ? Or am I being simplistic ?
I'm sorry but just adding range is being simplistic. Each tank adds a weight & drag penalty which reduces the effective range by a certain amount.
That being said, so is the statement of range for an aircraft (or almost any other vehicle/vessel/craft) without stating the over-all weight, temperature, humidity, altitude, speed, wind direction, et alia ad nauseum
As a rule of thumb I'd reduce the extra range by about 5% to get a more reasonable result.