HT V8 3spd is a close ratio box, but all the rest of the aussie 3spd's (HK-WB) are the same apart from input shaft length for V8. The front shaft seal changes late in HQ so the input shaft and cover change, and HX onwards boxes have a switch on the 2/3rd gear selector but these don't affect fitment to the car.

No V8 input shafts are the same as a 3spd. You can put the 6cyl or V8 3spd input shaft (except HT) into a 6cyl aussie 4spd M22 4spd though, but as there was no V8 M22 aussie 4spd the reverse isn't true.

If you have the stock V8 clutch rat-trap setup you want a Dellow or CRS 308TH (or Chev) to aussie 4spd bellhousing with bolt bosses for the push type clutch fork.

In a HK-HG this will also push the aussie 3/4spd backwards by about 3/4" over where it would sit normally as in a HK-HG the Chevs sit with their rear further back about 3/4" compared to a 253/308. So you'll have to shorten the tailshaft and move the gearbox crossmember which will probably affect the handbrake too. It may affect the column shift rods as well or make it not work properly plus the clutch setup may be affected similarly.

Due to the above it may actually be easier to try and buy a 308TH/Chev to aussie 6cyl 3/4spd bellhousing (Dellow or CRS used to make these) but the 6cyl aussie box is going to then move forward compared to where it should be and I think what you want is halfway in between the 6cyl and V8 box input shaft length - which is what a Saginaw or Muncie is. The right way to do it is to somehow shorten the V8 input shaft and cover to the same length as a Saginaw one and use a Dellow or CRS Chev to Sag/T10/Muncie bellhousing and have them drill the right holes for the aussie box mounting to it then the aussie box will sit where it is supposed to sit in a HK-HG.

The other way is to use HQ Chev mounts and shift the engine forward so its back aligns with where the 253/308 sits BUT you may then have steering to sump clearance issues, and possibly fan/shroud issues, throttle linkage problems, exhaust problems etc etc.

NOTE - I've assumed HK-HG above. If it is HQ-WB then the box won't move.

Also note, you can control an aussie 4spd with a 3spd column shift setup. You just have no reverse selection - you'd have to do that with a cable, but there is always the danger of selecting a forward gear and reverse together - you'd have to put the column in neutral before pulling the lever for reverse.

No, as 3spd T10, Saginaw and Muncie are all left side selectors. LHD LH Torana used a 3spd Saginaw for this reason.

The best way is to use a Powerglide. A HK-HT V8 column shift Powerglide and crossover mech should bolt straight into a HD-HR and use the HD-HR Powerglide column, just use a HK V8 powerglide crossmember.

On a HD though you'll probably be using a hydraulic clutch so the clutch mech will be OK with the Aussie 3spd box, and you are going to have to sort out the column linkages anyway. A HK 6cyl Powerglide crossmember would mount the 3spd all synchro in the same place in the HD where it would fit in a HK behind a 6cyl, but you'll have to push it back 3/4" for the Chev. What you might be able to do is make up a 6mm thick adapter plate that uses countersunk bolts with thread going upwards to bolt the plate to the mount, and another pair of countersunk bolts facing the other way 3/4" forward of the other two with their heads welded to the plate. These would go into the crossmember holes (there may even be enough room in the crossmember slots to get the 3/4" without even using the plate). If you had to use a 6mm plate you'd just put spacers under the gearbox crossmember like a HK V8 has to level the box again.

Simply shorten a HQ V8 tailshaft of some sort.

I'd also go and look at a Ford 3spd and see how long the input shaft is too as you could buy a Dellow/CRS TH308/Chev to Ford 3spd box bellhousing if the box's input shaft is the right length and the selectors are close to correct location. These will be easier to find as XF's still ran a 3spd. Probably reasonably strong too.

I have known blokes to drop a whole HK 307 and column powerglide into a HD-HR with zero mods other than exhaust and tailshaft. A HD-HR is essentially a HK underneath. So the 307 should drop in but as I said the 3spd has to either come forward if it's a 6cyl one or go back if it's a V8 one so you are going to have to deal with bits that get affected like column shift linkages, gearbox crossmember etc.

Cheers mate i already have gearbox sorted. Have everything sorted actually now need to work out a diff!