bull gears
FYI, the auto and manaul bull gears are different on the IH cases. That goes for the D20 and D300.
You probably know this, but the teralow kit is available for the D20 taking it from 2:1 to 3.15:1. The kit is pricey though ($600-$700). Advantage is the texas pattern D20 is readily available and cheap if you happen to destroy the case.
IH D300s are definitely rare, and you'll likely have to pay shipping to get one. Going price is ~$400.