Update:  I finally got my speedi sleeve installed. I had a delay because after I received the sleeve that I ordered, I realized that it was not wide enough to have both lips of the seal ride on the sleeve, so I ordered a second one. I measured and installed them so that they are just a hair apart and now each lip of the seal is riding on a sleeve. 
Due to rainy weather, I was not able to drive the car today. I am looking forward to seeing if this solves the ATF leak!

well I ordered a speedi sleeve today! Hopefully it will be here in about 5 days. I will report on how it goes for installing and if it cures the ATF leak. Thanks for all the info, I sure appreciate it. It sure makes working on this car a lot more fun when you have others for help and tips and to share in the excitement of fixing things!

stbart, 
There is an SKF speede-sleeve available for a 1-11/16 shaft.  The part number (designation) is 99167 as well as sleeves sizes on either side of it for slightly oversize or undersize shafts.  You should be able to order it from NAPA or other parts or bearing houses as well as McMaster Carr or Grainger.
I believe that the designation is a universal number that is used by suppliers as is, or they change it to a part number for their systems.  
The pic below is from the online SKF size chart. 
An accurate measurement of your shaft will make sure you get the right sleeve. 
Here is a link to one on Amazon.
 https://www.amazon.com/SKF-SSLEEVE-1-688in-Diameter-0-313in/dp/B00460FFQE

Thank you. I will be checking with Hill's. I can't seem to find a speedi sleeve for 1 11/16 inch shaft and I don't know if turning it down to 1 5/8 would be a good idea.

stbart,
Here is the number for the yoke you need, but you may want to check on a speedie-sleeve if the only problem is a groove in the shaft. I believe your shaft is 1-11/16. They are available 
This is from the 1949-1959 Mercury parts book. 
You may want to check with Hill's T Bird Parts.
There appears to be a conflict with the parts manual showing B6S 4841-A as fitting "all". 
Hill's shows 4841-C as fitting a FM (Merc-o-Matic).  Hill's should be able to help with that. 
  

I am sorry that I forgot to say, I have the Merc-O-Matic transmission. It is a 16 tooth spline. There is no speedi-sleeve on the yoke now, and I don't see any sign of one having been on there. I measured the outside diameter with my caliper to be 1.682. The 1 5/8 sleeve lists the diameter range 1.622 - 1.628. I will try to clean/polish it some more to see if the measurement changes.

stbart
It may depend which transmission and engine you have.  If you have a 312 and a manual it is probably a T-85, and if you have overdrive it should be an
 R-11 overdrive unit.  This is assuming the transmission is original or replaced with the correct transmission.  It may have been replaced with a T-86 which is a lighter duty transmission.  
If it's a manual there should be a cast number on the gear box opposite the shifting cover That says T-85 followed by a letter, usually a C.  The shift cover usually has a cast in T-89.  Ignore it.  You can also ID a T-85, by the curved bottom of the shift cover. 
If in fact it is a T-85 a driveline shop is a good possibility, and if they want more info make sure you have the yoke and tell them it's a Borg Warner T-85 and whether it has overdrive. 
T-85 yokes are getting hard to find.  I have both T-85 and T-86 transmissions on the floor and I can see if the yokes interchange.  I pretty sure they don't.
Let us know what transmission you have.  If it's a Merc-o-Matic yokes should be fairly easy to find. 
Another suggestion is to replace that speedie-sleeve.  I change them on a lot of shafts. They are supposed to be installed with a sealer under them to stop the leak you are seeing (usually red Loc-Tite).  
Take your yoke to a bearing shop and ask for the correct size sleeve they should have it.  NAPA also carries a good collection of  speedie-sleeves.  
Don

I have a yoke from a '54 Mercury which I believe should be the same as a '56.  If you are interested in it please PM me.  Alan S. Freeman

Check a local shop that makes drive shafts. They probably have one that will work.

Y's guys rule!
Looking for McCullouch VS57 brackets and parts. Also looking for 28 Chrysler series 72 parts. And early Hemi parts.

MoonShadow, 292 w/McCulloch, 28 Chrysler Roadster, 354 Hemi)
Manchester, New Hampshire

Hello, I am working on my 56 Mercury Montclair. I replaced the transmission tail seal due to leaking. After driving some miles it is leaking again. I notice that it is not leaking from between the housing and seal, it is leaking from between the yoke and the rubber of the seal. I removed the driveshaft again and inspected the yoke, it does have a wear ring where the seal rides. A couple guys had told me to replace the yoke. The problem I am having is finding a yoke specifically listed for a 56 Mercury.
Has anyone had to replace their yoke on this model car? If so, do you have a company name/part number?
If anyone has any suggestions, I would surely like to hear.
Thank you.