Hi,
Yes putting overdrive on a 140 is a good way to improve cruising refinement and, to some extent, mpg.
You need to look out for an M41 transmission, that is the 4 speed plus overdrive for the 140. The same gearbox was also used on Amazons with overdrive. They do come up for sale, e.g. on ebay, but they're not cheap, being both rare and desirable. Expect to pay £500 - £1500.
You also need a shorter propshaft. It's not too expensive to get yours shortened if you cant find one.
The M41 uses a different gearbox mounting cross-member. If you can't find one, your original can be modified to fit with a little fabrication and welding.
The wiring is straightforward. The M41 will come with an inhibitor switch so the overdrive only works in 4th gear. You need a simple on-off switch and relay to operate it, and a warning light
Good luck, its a very worthwhile upgrade.
What other upgrades are you interested in?
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