Re: What box for me??
I agree with the previous post. C6 is a really good option for automatic, it's also within the range of standard equipment. Let's face it, the relative rarity of even late 70's falcons (compared to the relative abundance of the mustang) will mean they will ride the wave that almost all australian cars of yesteryear have. Keeping your falcon in close-to-original-standard may pay dividends.
I had a C6 on my f100 ambulance (351cleve) which ran a tunnel ram setup, and the gearbox was stock(for an ambo), yet bulletproof. Not so the diff (9inch).
On the Manual end of the spectrum - it's got to be fmx. but you have to love manual too. It's no modern 5 or 6 speed, and traffic snarls will quickly end your love affair with the falcon.
If you are not afraid to depart from the original equipment there are heaps of mods and adapter setups for all sorts of gearboxes, especially on the 302 block basis. Go see a gearbox refurbisher and get some pro advice, or be super cheeky and call up classified ads with similiar age resto cars and ask them what they are running.