Where you going to put a 2 - 3 gallon oil tank? Where the passenger seat is?
Then you need to notch your front cross member.
You might be able to get some of the Stefs marine tanks where the AC box was, or have a custom tank made.
If you dont have a heater at all you can get something in that part of the fender fairly convienently. If you have something there, you might be able to get a cylinder style tank, probably about 8" in diameter and 16" top to bottom.