I am in the process of installing a Mcleod RXT twin disc on my Challenger with a TKO600. There are a couple people on here that gave it good reviews.
Mcleod makes 2 versions a RST and RXT. RST has organic discs and the RXT has ceramic discs similar to their 600 series disc.

http://www.mcleodracing.com/products/RXT+Twin+Disc.mcl