I have a built TH400 that I built for my Sonoma SFA project that I likely will abandon. It still have 0 miles other than dyno tested at my college when I was in my community college majored in automotive service, built ~3 years ago. Not guaranteed but was dynoed good. My instructor, who owns and run an auto trans shop (builds lot of tranny for high HP vehicles, mostly drag cars) and teach at nighttime at the college. He said the tranny I built will holds 800 hp max (IDK how true to this but plenty nonetheless).
IMO TH400 or 4L80 is way to go if you want it to last, especially for the money.
let me know if you're interested in this and I'll sell it to you at a good price.
Anyhow, if you need some help to get it fixed like get the tranny pulled, or so, I'll like to help as I currently have nothing but time.