[Kahuna]

Roderick
The speedometer comes off the OD, so no adapter/reduction is needed. Just a custom cable.
The best way to handle the reverse lock out would be to use a switch
located in the shift tower itself actuated by engaging reverse.
I don't know how it would be done on a Model A, but someone has done this, I'm sure.
For my car, I'm using a different transmission case and the shift tower has a backup light switch that I used to disengage the OD.
On my car, a 32 Ford, to wire the OD ( A Gear Vendors unit, similar to the Volvo stuff), I used a 60's OD handle and mounted it to a 2 position push/pull switch with an LED so that when the handle is pulled, the led illuminates and the circuit is completed so that the OD MAY be engaged (kinda like an arming switch). Then I have a momentary switch installed in the shift ball (A Volvo shift ball), to actuate/engage the OD. To do this, I bought a Marlin Model 66 .22 cal rifle barrel and ran the trigger wires down thru the barrel, exiting just before top of the transmission tower ( the rubber boot covers it). I had it chromed after welding and no one can tell the difference unless they know what to look for.
I assembled a set of 5 small Bosch relays to create a latching relay system, so that hitting the momentary switch engages the OD, hitting it again, disengages the OD.
This works real well, gives great mileage, etc.
You'll love the OD
Jim