I think I may have achieved Grand Guru status!

I have had a win drilling and tapping the rear or the extension housing and fitting a new reverse switch. It is actuated by the END of the rod which normally strikes the side of the factory switch

Slapping a B310 5 speed into a 1200 or 1000 is not real difficult but the clearance (or lack thereof) for the factory reverse switch makes keeping the switch a real pain.
there are generally 3 options:

1 plug the swich and mount a new switch
on the dash (dodgy)

2 plug the switch and fit another next to
the gearstick--ala Levey

3 take a 40 mm hole-saw to the side of the tranny tunnel and screw in the switch after installing the gearbox

or drill and tap

the switch I used is from a landrover, It is 9mm diameter and has 20mm available thread
thread is 3/8UNF 24tpi
Cost a huge $20 AUD

I have only experimented with a FS5W60L dogleg box but it should work on the FS5W60A box as well however a relay would be req because the operation of the 60L & 60A switches is different

PS while hunting for a suitable switch , I found a replacement for the Suzuki Sierra which looks identical to the factory (Niles) switch on the dattos
Also $20 on ebay