Many thanks everyone for the discussion and sharing the common traps.
I've now got the gearbox set up for an on-off type switch positioned on the gear lever with plans to convert to a momentary switch with latching relay arrangement in the future. This seemed like the simplest method to be able to drive/test the car as I'm considering changing diff ratio also. I made sure to carefully check the inhibitor switch on the gearbox and it only allows current flow to the overdrive solenoid while 4th gear is selected and the overdrive switch is in the on position.
@Anthony based on the wealth of discussion here, a B20 to M46 mechanical guide could be its own thread! If someone has a source of the factory B20 to M45/6 bellhousings I'm sure there would be a lot of Australian interest.