I been reading some stuff too. Have you seen this? I think the ratio will depend on the type of cam is in your engine and what driving you will do. Do you want good highway cruising or good takeoff.
https://sites.google.com/site/modifingfxfjholdens/rear-endAnother article worth a look. Given that hr had 13 by 6 it would make for a good town ratio.
http://holdenpaedia.oldholden.com/Differential_Ratio_selection2.99 was first gear ratio on a grey manual crunch box.
Trimatic
So usually you can run a higher first gear ratio like the trimatic has if comparing to the crunch box cos it's an auto. I think it should work well if your just doing it to match the auto better but you will still be reving fairly high on highway as per the example in the link, unless you have 14" wheels because the top gear ratios are all the same.
1:1 ratio top gear (3 speed gearboxes) the 3.89 gives 3400rpm (29kmh/1000rpm), the 3.55 3100rpm (32kmh/1000rpm) and the 3.36 2900rpm (34kmh/1000 rpm). Personally for an all rounder, given that banjo centres are same up to late 70's , with a traumatic in an fc (being rather light) with a red motor, I'd choose 3.08 or 3.36. But with a grey you could go with a 3.36 for highway but a 3.55 I'd think is a better all rounder remembering that the first gear in the trimatic has a higher first gear ratio anyway. So may become a bit sluggish off the line with anything taller than a 3.55. Another thing you could do is slightly change the stall of the gearbox so that you can run the highway diff on a grey (3.36 or taller, noting that with a small red Torana had right up to 2.78 ), but still retain good takeoff. Ok. I'll stop waffling on.
