This may have been covered before. I have also spoken to gm5735 on this at length as well. But I am still confused.

On the surface it looks like simple geometry: GMH simply used a taller mount adapter on the driver's side to effectively tilt the SBC design engine so it fitted (in combination with the sump and passenger side exhaust manifold). As far as I am aware there is no difference between a HD-HR and a HK-HG with regards to where the engine/suspension crossmember has its engine mount stubs welded wrt to the centre line of the car and wrt to being aligned horizontally. Basically there is no apparent offset in the engine bay of a 6cyl or a 253/308 in one of these cars which you'd expect if the actual welded on mounts were anything but totally symmetrical.

HOWEVER. When I approached both the makers of Tuffmounts and the fabricators of the aftermarket gearbox crossmembers both gave me a big spiel how the engine is tilted and offset in the engine bay? My replay was "yes I know that, but I want to centre it hence why I'm trying to buy 2 x passenger side Tuffmounts and a gearbox crossmemebr with the transmission mount centred". Apparently it cannot work that way?

Right now I have a 307 and Powerglide in a HK GTS up on the hoist. It actually has 2 x tall (driver's side) adapters in it with an original HK V8 Powerglide gearbox crossmember and rear mount. The engine almost sits level going by the bottom of the sump. I'm pulling the driveline and crossmember out soon, and I plan to stick a bare SBC block and TH350 in there on the Rodtech front crossmember so I can get the steering shaft sorted and find extractors, block huggers etc (even Corvette 2.5" Ramshorns) that fit it properly. I will just stick 2 x short (passenger) side adapters in there and move the TH350 across on the factory rear mount to centre it so I can see how it all looks. However I wanted to at least order an aftermarket gearbox crossmember with the box centred so I can look at exhaust clearance for dual 2.5" pipes, and not get grief from the supplier about it not working etc.

So just wondering if someone has actually levelled all this up and taken measurements as to whether it is really as simple as I think or if there is some other hidden trickery going on as per the Tuffmount maker's claims? Note that I want to actually use 2 x short side Tuffmount mounts eventually, but if they do not place the engine as per original this may not actually work. I did notice there were some others around too like Rodtech if Tuffmount don't work, Rodtech's sit claims they sit the engine in the factory position so I should be able to use 2 x short ones of their design.
https://www.rodshop.com....type-engine-mounts.html

Lastly, do HK-HG with Saginaw or V8 Powerglide use the same rear mount as an Aussie 3/4spd or Trimatic? The Tuffmount rear mount description says they have a shorter one for TH400 (which I know in HQ-HZ is shorter than the 3/4spd or Trimatic mount) but say the V8 Powerglide one is common to Trimatic and Aussie 3/4 speed. Is the TH350 and V8 Powerglide plus Saginaw mount shorter as well or is it the same as a Trimatic one? I've never really taken notice before, but a HZ parts catalogue doers not help as 6cyl and V8 3/4spd and Trimatic are different part numbers to each other, TH400 is different again and so is TH350 different number to all others. And there is some weird stuff with HT where the mount used on V8 Powerglide differs between 253/308 and 307 and 350 (all different) but it does appear that Saginaw 4spd and Trimatic use the same rear mount for 253/308 and 6cyl. What s very interesting though and probably answers my question is HK Brougham uses the same rear mount as 6cyl Powerglide 7432236 which is the same as HQ-HZ V8 3/4spd and V8 Trimatic, yet the rest of HK V8 Powerglide uses a different rear mount 3870184 which is the same mount as HK 307/327 4spd and the same as HT 307/350 4spd or 307 Powerglide. So I guess if I use full height 743226 mount under the TH350 on the I will need the spacers between the x-member and the body, but if I use the shorter TH400 mount I can delete the spacers and thus have a tiny bit more clearance around the crossmember, bearing in mind it will be one of these:
https://www.ebay.com.au/...d4761:g:2UMAAOSwfO1cy27U

Edited by user Tuesday, 14 May 2019 2:20:30 PM(UTC)  | Reason: spelling

As far as I can tell the only difference between the various rear mounts is in the 'stiffness' of the rubber. Obviously there are exceptions like the TH400 & the Opel etc., but many of the others are interchangeable.

In other words many will fir other applications, because they are the same physical size, but some will transfer more or less vibration through the mount than others.

There are several different part numbers of rear mounts which all cross over to the one Mackay mount for replacement purposes.

Dr Terry

The tailshaft is also off-centre at the diff, because the axles are of equal length, placing the pinion to the right.

Dr Terry

This is the trick to making a good tailor-made 9".

If you make the 2 axles of equal length as the old Holdens were, the 9" has so much pinion offset that it places the tailshaft even further off-centre, so far so that in early Holdens the rear uni hits the side of the tunnel.

If you make the axles unequal in length (the same amount as in a Ford), then this places the tailshaft dead-centre, again most likely hitting the tunnel on the other side. This is because the Holden tailshaft tunnel is off-centre

You need to do some calculations to place the 9" diff yoke in the same position as a Holden diff (i.e with less offset).

Dr Terry

The figures that I have put a stock Holden diff at 3/4" (19mm) off-centre & a 9" at 2" (51mm) off-centre. Assuming equal length axles in both cases.

Dr Terry