why not use the original throwout bearing and clutch arm. I take it you changed bellhousings with the engine change.
I know later on the bearing retainer was used to index the transmission to the bellhousing. On gm trucks from about 48-67 the bearing retainer eas 4.686 inches in diameter, and the snout that the throwout bearing rode on was 1.373 inches. My 1940 GMC transmission is indexed to the bellhousing, but I am not sure of the size.
Food for thought... When a later transmission like the A833 or the SM465 is put in an older truck, an adapter plate is used or the bellhousing is bored out for the 5.125 bearing retainer. If you go the other way, lets say put a sm420 in a truck that had a sm465, the bearing retainer is too small. To remedy this, a "donut" is used. The donut presses into the bellhousing and then indexes the bearing retainer.
In your application, using the later bellhousing, you can probably have a donut machined out of aluminum to index your transmission. Once you have the donut in place, you can redrill and tap the mounting bolt holes. You might have to roll the transmission slightly to the left or right to get a clean spot to drill. Since you are using the stock transmission, the stock throwout bearing should work. The Monkey wrench in this scenario is the newer bellhousing and clutchfork. I am not sure about the new geometry invvolved here.
John
P.S. I'm also in Northern Cal
