If you use a CRS Tunnel you can get an EK Auto molded carpet to fit and you can still have the bench seat too. If you want to mess around you could investigate tunnels from other cars that would still allow for a molded carpet, but IMHO you're better off with the CRS jobbie.
If you are going for R&P Steering and want a column shift auto then a Torana column should work, a nice simple column without all the plastic and extra controls on a Jap column.
Rod's Racks has a good rep for R&P conversions, best to speak to them about PS.
Airbags
I'd be checking with rego before going any further, I know in SA they are frowned upon.
Instead of the Commo diff have you looked into Volvo gear? A diff out of a Volvo 240 is about the right width (a little shorter from memory) and is a Dana 30 (really thick axles), disc brakes and Old Holden stud pattern. The only mod needed would be to weld new mounts to it and they are dirt cheap.
Generally you get low gears eg 3.73 and 4WD Systems (SA) make a relatively cheap locker for them.
Getting rid of the rear leaf springs is really getting into the "Pro" territory now, no doubt engineers will need to be involved. There's a bloke building a SBC, tubbed and 4 link rear EK Sedan on the FB/EK forum, lots of pics and definitely worth a look. Places like McDonald Bros advertise rear clips for 4 links, but I think you'd have a tough time registering the car.
