Overdrive Unit

[Barrie]

The Jensen Forum route suggested below is a good idea.
I investigated Gear Vendors and discarded the idea because the shortened prop shaft would have restricted movement of the rear axle causing handling problems. This MAY not be so severe with a Bristol because of the longer wheelbase compared to my Jensen.
I replaced my CV8's Torqueflite 727 with a later 4 speed with torque converter lock-up 518.
Gears 1 to 3 are roughly as the 727 so you still have the previous performance but at more than 40mph the car drops into 4th then torque converter lock-up giving 38mph per 1000 rpm (3rd gives 27mph). It's very relaxing, quiet and economical. Less wear too. It transformed the car.
I bought the 'box from USA as second hand rebuilt from a reputable source and the cost 7 years ago was £2200 plus shipping. Fitting cost c.£2k and was quite difficult but a 411 will be easier as there's more room. You need to wander around the internet looking at USA where there's a wealth of up to date info on the subject.
Good luck!