Series 3 Fairey Overdrive pre drive check?

Hi, I’ve just installed a Fairey OD and before venturing out and risking destroying anything should I be able to engage the OD in and out when stationary? If so is there a method?

Currently the Selector Shaft is ‘out’ which I think is disengaged (not fitted the ‘gear lever’ yet). But using the Link Rod I can’t move the selector shaft in.

Any thoughts appreciated.

It's a synchro gearbox. Needs rotation for the snchros to work. But getting the lever right so that the detents and the cogs are in alignment is important.