Transmission pops out of third gear

[TJMack]

I looked back through old threads on this subject and found reference to shims between the bell housing and the engine block. I want to start with the simplest solutions first but I need a little guidance on this.

How thick are the shims supposed to be, and do I understand that they go on the two bolts that hold the throttle linkage to the engine?

The car (1931 Tudor) is new to me. The previous owner said he had a new clutch and balanced flywheel installed, so I'm suspect that the shims may have been left out.