518 short shifts and no od

If it is a non lock up unit you need the 2 wire connector just behind the manual linkage hooked up for overdrive.
The 1-2 shift is correct at 20 mph at light throttle an 3rd should be about 30 mph or so.
If you let it shift from 1-2 then mash the throttle to the floor it should down shift to 1st gear at low speeds like 25 mph.
If it is shifting 1-2 every time you take off from a stop it is not the governor.
How do you have the overdrive wired up?