if you can,t source an over drive theres another way to do it , most Plymouths came with a 4.1 diff , but there were other options available that fit , you might have to go to a later model , or dodge chrysler or desoto , they were 3.75 or 3.9 , also another option is , if your car is not going to be judged (you need a freindly engineer, fitter turner )is remove the tail shaft , and have a more modern electric over drive fitted , i did with my 54 , makes a big difference in the engine revs , therefor less gas , and higher croozing speeds

Last word as of about three weeks ago was that George was Currently out of stock on OD's.  There is one Buy it now on ebay but it looks like a rattle can rebuild, and I wouldn't touch it with a 10 foot pole. 

I contacted the seller with some questions and he refused to answer through the ebay channel.  Suggested I give him a call.  Must be afraid to put his answers in writing. The first offering was a  no reserve auction, but started at 1500, so does't that seem like a reserve???  When that brought no bids,  he re listed it Buy it now for 1600. 

My guess is that it's nothing more that a core that needs work. 

Might also try Neil Riddle,

Pete

for an over drive try    Asche mechanical ldistributers, George Asche , 1693 Fertigs road , Venus, Pennsylvania 16364, (814)3542621, sorry this is the only contact info i have , and dont know if it is still curent , but its worth a try , he specialises in repairing and selling mopar overdrives (or did)

Trannies are a bolt in swap, with a bit of wirining for the kickdown switch and the governor which determins the kick in speed has been met.  they are running 900 to 1500 depending on condition if you can find one.  You are looking for a Borg Warner R10 for mopar.

You might wnat to look at changing the rear end, to something like a 3.55 ratio.  As florida was last time I looked kinda fflat, you son;t need ot be concerned about hill climbing.  Lotsa candidates, Mopar B body ( chargers, satilites coronets) or Jeep small cherokee, or lots of stuff from fords. All will run the stock lug bolt pattern. need to cut of the old sprig perched and add new ones (folks are using trailer axle perches from northern tool) then adapt the driveshaft/universal.  Same result less expense. taller tires help also.  I am runinng 225 r 15's.  Running my gears and tire diameter show 3280 Rpm = 65 mph, 400 rpm belo the horspower peak.

I run my 46 with its 4.11 rear end all over the place at 65 indicated.  Have made runs from central NY ot NC, Detroit, Va, Vermont, Etc.
So you can run your 55 a bit faster.  Remember most of the noise if coming from the fan.  As long as you have good oil pressure, and good cooling I think you can get a few more MPH out of your current set up.  I am more concerned with my brakes than running at 65.

I have a 55 Plymouth Belvedere 6 cylinder with a 3 speed manual transmission. I live in Florida where the speedlimit on I 95 is 70 mph. Going along at 50 mph you are almost standing still.
I am considering changing to an overdrive set up. Does anyone know where I can get one and what is involved to convert it? My engine will thank you.