I'd go the trimatic. Easy enough to modify to handle more power, easy enough to convert to manual shift, good range of after-market products for it.
You'll have to re-do the gearbox to fit the auto. it sits further back and i believe the output splines on the auto are different to the manual.
Bit of a shame your running in the stock class otherwise you could also hook up a second serria hi-low box for an extra, extra low gear.