Cub, I have been doing overdrive research myself, including calls to various manufacturers. Gear vendors stuff if neat, goes behind the existing tranny and actuates electrically. Advance Adapters makes the Ranger unit in a overdrive (27%) or an underdrive. These units mount between the bellhousing and existing tranny and actuate via a customer supplied shifter. They are about half the price of a Gear Vendors unit. They have a bolt up unit for the SM420 tranny. Do a Advance Adapter search!!
Atomarc.