I swapped in a Richmond 5 speed and had the same problem. I had to have the bearing retainer turned down to the proper dimension.
I'm getting about 1.357" OD on my original retainers. A new throwout assembly is 1.365" ID.
The complete bearing assembly is:
Group 0.799 pn 1367058
Used on 1939-48, 53-60, 63-66 Nailheads
Also check the OD of the bearing retainer flange, I had to have mine turned down to fit the bellhousing. Here's a pic of the Richmond vs the original T-10: