I've got a Level 3 700r4 from Bowtie Overdrives. It includes all the performance mods as well as the hardened parts they call "Extreme Duty".
Admittedly, I spent a lot of money on this tranny (almost a grand on the tranny, plus 200 on the torque convertor, the stacked plate cooler, the temp guage, etc...) all said, i've got almost 1500 in the whole install to replace the one I burned up by my own mistake. I knew it was low on fluid and ran it hard on the rocks anyway. Didn't realize it was almost 5 quarts low /forums/images/graemlins/eek.gif /forums/images/graemlins/yikes.gif (yes, i'm a dumb ass).
If I knew then what I know now... mainly that I no longer plan to drive this thing on the road, i'd have spent a couple hundred on a TH400, and built it up if it ever died. In fact, I now have a TH400/208 sitting in the garage as a backup if I ever blow this thing up.
77 Mud Buddy (my neighbor and partner in crime) has now gone through two TH350's in his K5 and he recently installed a stock TH400. He also bought a K30 dually for the axles before deciding to use it as a daily driver and the TH400 in it was junk, so he bought another for it, and this one (also stock) is headed south quick, but if anyone can break something, he can!