The standard Gear Vendors ODs all have an adapter that replaces the stock trans tail-shaft housing, and a splined female coupler with the selected transmissions spline on one end, and the standard GV spline on the other. The output end of the GV is a fixed yoke for the universal joint, so the drive-shaft will need to be modified with a sliding joint as well as shortened by approximately 20 inches, IIRC.
The OD is activated with an electric solenoid, so it can be manually controlled with a simple toggle switch; the control is only needed for automatic operation.
My first experience with an over-drive was in 1966, when I was 12. Ron, the neighbor across the street had replaced the clutch in his Triumph TR4A IRS, and asked me to push in the clutch pedal while he was underneath adjusting it. Afterwards, he took me on the test run, and told me the car had 8 speeds, then proved it by counting off the gears.
IIRC, there was a stalk on the right side of the steering column the switched the OD in or out.