Figure I'll throw my 2 cents in here
I would strongly suggest investigating the 4l80e some more. I went through 4 700r4's in my '84 behind a relatively mild 406, so in my '64 I'm going to try out a full manual 4l80e. I've done a ton of homework online and almost everywhere I've looked claims that a mildly upgraded 4l80e (no major hard part upgrades) can handle nearly 900 hp. My plan is to just freshen up my 4l80e ($500 ebay score) with new clutches/seals/etc. and use the Transgo 4l80e-HD3 full manual kit. The cool thing about the Transgo kit is it uses a vacuum modulator mounted internally which allows the line pressure to be load dependent so when you're putting around town you're not barking the tires all the time (it's fun barking tires, but not good for a daily driver
). By my figures I should have less than $1500 in it and it should be good for at least 600-700 hp.
Just my 2 cents