They can take a lot of abuse. I have about 15 to 20 track passes on mine. Leaving at 6000 and Power shifting at 5800 everytime. I have not had any problems
I have the older TKO. I bought it used from a dude on here. You need a 26 spline clutch disk (might as well buy a new clutch kit) a 31 spline yoke (might as well buy a 31 spline aluminum driveshaft) 3 quarts of GM syncromesh, new shifter, I think something has to be done to the xmember also, but after looking at mine, it doesn't look like anything was done to it. Get a new trans mount too. I didn't do the install. But I recently pulled my engine and trans so i'll be re-installing it this time myself.
In 5.0 mag a guy had a 700rwhp fox with 5.4 Navi motor and turbo......he ran a 3550. Another guy had a 408 with n20, he made 800+rwhp and he also used one.