I've run the numbers with those specific tyre sizes for you Howie...
Tyre size = 285/35R18
Tyre diameter = 657mm (25.85")
Tyre Revolutions = 484 per km
I don't know what speedo cable drive/driven gears are available for a T56 Magnum, but I can give you a ratio between the output shaft and the cable for a standard Torana speedo for your tyre size with a range of diff ratios and you can work it out with the supplier from there:
Output shaft : cable
3.00:1 diff = 9:21 (reads 99.5 @ 100)
3.36:1 diff = 9:23 (reads 101.7 @ 100)
3.55:1 diff = 8:22 (reads 99.9 @ 100)
3.70:1 diff = 8:23 (reads 99.6 @ 100)
3.90:1 diff = 8:24 (reads 100.6 @ 100)
4.11:1 diff = 7:22 (reads 101.2 @ 100)
So for example, a 9 tooth drive gear with a 21 tooth driven gear. If you go with a diff ratio or tyre size other than any above just let me know and I can run that too?
Now the engine RPM at 100km/h for a selection of diff ratios in 6th gear, firstly for the TUET11012:
Gear Ratio = 0.50:1
Diff Ratio 2.60:1 = 1049 engine RPM @ 100km/h
Diff Ratio 2.78:1 = 1121 engine RPM @ 100km/h
Diff Ratio 3.08:1 = 1242 engine RPM @ 100km/h
Diff Ratio 3.36:1 = 1355 engine RPM @ 100km/h
Diff Ratio 3.55:1 = 1432 engine RPM @ 100km/h
Diff Ratio 3.70:1 = 1492 engine RPM @ 100km/h
Diff Ratio 3.90:1 = 1573 engine RPM @ 100km/h
Diff Ratio 4.11:1 = 1658 engine RPM @ 100km/h
And for the TUET11009:
Gear Ratio = 0.63:1
Diff Ratio 2.60:1 = 1321 engine RPM @ 100km/h
Diff Ratio 2.78:1 = 1413 engine RPM @ 100km/h
Diff Ratio 3.08:1 = 1565 engine RPM @ 100km/h
Diff Ratio 3.36:1 = 1708 engine RPM @ 100km/h
Diff Ratio 3.55:1 = 1804 engine RPM @ 100km/h
Diff Ratio 3.70:1 = 1880 engine RPM @ 100km/h
Diff Ratio 3.90:1 = 1982 engine RPM @ 100km/h
Diff Ratio 4.11:1 = 2089 engine RPM @ 100km/h
Here's a couple of 'visual aids' with a 3.36:1 diff used for example (the speeds will obviously change for other diff ratios) and a shift point of 5000 rpm, firstly for the TUET11012:
Compared to the same shift points for the close ratio TUET11009:
You can clearly see (looking at the lower graph on each picture above) that the closer ratio gear set of the TUET11009 has a much nicer "even" spacing. 2nd gear is almost "optional" in the TUET11012.
And finally, driveshaft RPM @ 120km/h (cruising speed for ½ critical RPM):
Diff Ratio 2.60:1 = 2517 driveshaft RPM @ 120km/h
Diff Ratio 2.78:1 = 2691 driveshaft RPM @ 120km/h
Diff Ratio 3.08:1 = 2981 driveshaft RPM @ 120km/h
Diff Ratio 3.36:1 = 3252 driveshaft RPM @ 120km/h
Diff Ratio 3.55:1 = 3436 driveshaft RPM @ 120km/h
Diff Ratio 3.70:1 = 3582 driveshaft RPM @ 120km/h
Diff Ratio 3.90:1 = 3775 driveshaft RPM @ 120km/h
Diff Ratio 4.11:1 = 3978 driveshaft RPM @ 120km/h
And again @ 240km/h (theoretical top speed for critical RPM):
Diff Ratio 2.60:1 = 5034 driveshaft RPM @ 240km/h
Diff Ratio 2.78:1 = 5382 driveshaft RPM @ 240km/h
Diff Ratio 3.08:1 = 5963 driveshaft RPM @ 240km/h
Diff Ratio 3.36:1 = 6505 driveshaft RPM @ 240km/h
Diff Ratio 3.55:1 = 6873 driveshaft RPM @ 240km/h
Diff Ratio 3.70:1 = 7163 driveshaft RPM @ 240km/h
Diff Ratio 3.90:1 = 7550 driveshaft RPM @ 240km/h
Diff Ratio 4.11:1 = 7957 driveshaft RPM @ 240km/h
You need to know the critical speed for your exact driveshaft and stay below it, you also want to avoid sitting around ½ of that critical speed under normal driving conditions so you don't have excessive driveline vibrations.
Driveshaft critical speed is the speed at which the harmonics meet the natural resonance and the shaft shakes itself to pieces. Critical speed is determined by the length and diameter of the driveshaft, as well as it's rotating mass. The shorter the shaft, the higher the critical speed. Same goes for the diameter, bigger is better. And light-weight materials such as carbon fibre make a big difference too (although there are other considerations with that option too).
You really need to crunch the numbers on the particular driveshaft that you'll be using but as an example, a standard 5.0L V8 Torana driveshaft (2.75" dia, 49.95" long, 1310 unis) would have a critical speed of 7192rpm and a ½ true critical speed would be 3589rpm.
If this was the driveshaft you're going to use, you can see from the info above that you couldn't really go lower than a 3.70 diff gear, but again, with a T56 your driveshaft will likely be shorter and so will have a higher critical speed.