Diff's

[john]

bedford diff housing is1568 mm flange to flange and 1728 drum to drum, vs commodore is 1455 f to f and run a 722 mm short axle and 820mm long axle, so you cut short side of housing, sleeve and extend to run 2 long axles, now 1555 flange to flange. disc to disc is1677 so to get  tracking same as front you change offset of commodore rims to hq back space.   or restud pcd of axles to hq and run hq rims.

[rumax]

If you are going for a VK EFI. you will need to modify the engine bay a bit to fit it in. Also, don't forget, there are options of fitting a 4 speed holden auto or 5 speed celica to a red motor.... and keep Regency happy...  (regency is the inspection station for all those non- adeladians).. As far as I know, as long as the engine is the same type, there should be no issues, just an identity check prior to rego. And that does not require a pit visit... as long as it LOOKS LEGITIMATE!! Anything that look dodgy, they can "require" you to have a full inspection. What I do is have everything nicely "marked" to make their job easier...like highlighting the engine/body number - chalk works well.... and, well, you know what I mean. Make it look standard...quiet.... an...um...unobtrusive... and they will sail it thru! And if it has SA rego already, well, it's just walk in! - I don't think having EFI is an issue... and they came with 202's.

As for diffs... well, that depends on what your running...tyres/gearbox ratio and engine power band. My traytop runs a 3.5 diff with a 186/trimatic... and I am fiddling to get better drivability- apparently it has 100 ish rear wheel HP, but my economy sucks!. 
I run the standard drums and find them acceptable- never had fade. All it means is that I don't get 10 feet from the car in front before I brake!

Cheers & good luck... Russell