I have one behind my nailhead (64 'lark with T-10 wide ratio and 3.54)
This was after looking at the cost for a 5 speed and as Tom mentioned, I could always use the unit behind anything I want, with only having to buy a correct tailshaft housing for trans type and the speedo sensor for gear specific gear ratios for it to shift at the correct MPH in gear(s).
It is a pretty easy install and simple to operate.
Before I was running about 3600 RPM at 70-75 and now it is about 2600.
And it is a hoot to drive around town, and being able to select over/under in any gear (especially 2-4) is touch of button rather than shifting, so it is fast and direct.
Digging and hitting the button on launch, it autoshifts to first and it really pulls hard and almost 30% further than 1st alone.
Driving around in 3rd over, is a bit higher than being in fourth, but "downshifting" to third while making the loud pedal flat will get about 30 foot of squeal and some wide eyes.
I am very happy with it, and the nailhead torque seems to be pretty happy no matter when or where I hit the button.
(Yes Edouard, it would make pulling a trailer a delight behind the billy-goat, or just plain old fun hauling apples)