By the time you upgrade and/or replace all of the wear parts in a D20 you're well on your way to an Atlas. Don't have to buy it with crazy low range gearing. I went with 3:1 on mine. Should I need more, which I highly doubt, I'll go with a doubler instead of changing the ratio. With a 130" WB that's an easy choice for me, a std WB might think differently.
Speed & HP favors bigger drive-line parts. I'd say 1330 min.
As ntsqd said it might be time to think about an atlas, but I upgraded all the gears in my D20 for about $1K (had a shop do it) and have been very satisfied. The D20 is a pretty bulletproof little t/c, and because it's short I get the extra travel (19" rear) because of the longer drive shaft length.
I would go 1350's with the torque that motor will put out.