th400 with out a doubt
4l80 4th/OD is the week link.. wieght beings and issue
the NX dmax stang is pushing 9 sec 1/4s wieghs 3300lb w/o driver and runs a full built 4l80 manual vb with trans brake
Go 4l80e, instead of th400 you can tune it not to go into overdrive at wot, thats what all the 60e truck guys do untill they can get a 80e swap. Then just gear it right and wala! you have a 8sec 30mpg truck!