Most ODs have a 4th gear switch so you may not hear the click if the
shift is not in 4th.

You can tell best while driving in 4th at 40 or 50 mph. When you
do the switch you should feel it shift into OD and hear the engine
slow down.

If it doesn't engage, look under the car and see if there is a football-
sized case between the transmission and the driveshaft. That is the
overdrive. It is possible that someone has replaced your M41 tranny
with an M40, which doesn't have OD and uses a longer front driveshaft.

If you are not getting Rolling mag, you need to. It has great articles
and ads from LOTS of parts suppliers.
--
George Downs, The "original" Walrus3, Bartlesville, Oklahoma