The 6R80 is twice as strong as that 4R70W. You'd be crazy to go with anything else. Beefcake is making 1100HP with his, just as one example. And it would be easy for you to do as they come factory in Mustangs just like yours.
I agree . But.... no baody makes a conversion from mt82 to 6r80. It would take twice as much money to convert to a factory auto then a 1500 hp capable 4r70.
The super low first gear in the 6r80 allows you to keep mild rear gears (3.15, 3.31, etc) so you get great mileage on the street, but still 60' awesome at the track.
The up side to the 4r70 is the ability for a transbrake. Have seen some people complain about the 4r70's holding up with lots of power and heavy cars though. Lifetime warranty is great, but still sucks if you are pulling your trans and shipping it somewhere a few times a year for the "free" warranty work.
If anyone from Chicagoland has the know how and desire to swap manual for an auto I'm game. I miss driving a manual car.
What year is your car? Miles? I know someone who might be interested.