Also check the OD of the bearing retainer flange where it bolts to the trans.
To install the Richmond 5 speed into my '66, that also had to be machined down to fit the Nailhead bellhousing.
A quick measure of the retainer shaft for the Nailhead gives 1.357". I can double check if you need a precise measurement.
Here's a pic of the Nail vs Richmond retainer, showing the larger flange: