I have purchased an intermediate handbrake cable for my HT and it appears to be too short by about 18 cm (with the adjuster fully extended).
Can anyone tell me what it's overall length should be as mine is approx 180 cm (6 feet).
The cable which came out is somewhere in my garage and was removed years ago so I don't have it to compare and if I had it I wouldn't be in this situation anyway!
Also does the length vary between sedan, coupe, wagon, etc?
Another possible complication is my rear axle is an HQ Salisbury.

I measure an original one at about 1835.
Perhaps the HQ diff cables are shorter. The HT Salisbury cables are about 915 from the grommet face on the rear of the backing plate to the outside of the ball end.
If the HQ cables are shorter I guess you could replace the cables, or make a longer threaded pull rod to compensate.

I agree with your assessment GM5735, and I came to similar conclusions while I was under my car in the last few days. The car has been assembled with a different rear crossmember (as the one which was previously there was poorly modified) and it appears to fit my PG transmission and the rest of the car quite well. I also have a spare standard HT manual crossmember so I will check dimensions, however I don't think this is the complete answer as there's quite a gap to close up. Could I have done something wrong with the rear drum brake setup?

Does it have HK-HG cables on it (drum end) or HQ to WB?. The backing plates are essentially the same as HQ-HZ so the cables will swap over, this may fix your issue.

V8 powerglide and Saginaw crossmembers should be the same basic thing (shifter mount aside). They are for HK. 6cyl 'glide is different.

The x-member on the car (and the other 2 spares I have) is 180 mm wide and the clevis point is 75 mm from it's back edge (the previous x-member in the car was 110 mm and the standard 3 speed manual x-member I have measures 75mm). So the current pivot point is short by at least 35 mm compared to the previously installed one.
Installed x-member sits between the PG pan (with a 25 mm gap) and a pair of mufflers (with 50 mm gap). So may be able to move the x-member rearward by 45 mm toward the mufflers.

With the intermediate cable installed on the left/right axle cables and the equalizer fitted, the equalizer is 90 mm away from the end of the (130 mm long) threaded hook rod. As the equalizer would at least be, say, 30 mm engaged onto the rod, I am currently short by 120 mm (vs my previous estimate of 180 mm). Pulling the equalizer to operate the rear brake mechanism provides about 20 mm movement of that 120 mm.

After looking at this again, it looks like I have to remove the gearbox x-member and extend the handbrake pivot extension by 110 mm to get the handbrake lever and cables in their correct position. However, apart from the fact that the current pivot extension is 35 mm shorter (75mm vs 110mm) than the previous one installed, I don't understand why it has to be extended more than 35mm since the current x-member is in the same position it was previously. Maybe previously the handbrake lever wasn't properly oriented but the handbrake worked fine. Thanks all for your assistance.