Just finished installing an m42 overdrive in my 1963 544. What a change! It dropped almost a 1000 rpm at 65mph and my gas mileage is over 30mpg! It is not an easy task which involves cutting the trans tunnel and fabricating a wider space and cover in addition to cutting the drive shaft and rebalance. Suggest installing an angle drive for the speedometer cable as it is too severe an angle change without.
I am going to do this to my 64 (544) It has a B-20 motor already but the 410 gears make it difficult at 65.
Thanks for your great pictures.
Where did you get your transmission?
Anything else that I should be aware of before entering into this?