You really need to evaluate your situation. It seems really easy to make insane horse power that is manageable with EFI and other technological advances. With that statement, you have to determine how you need to get all that power to the ground. Your car has to be a package, not a hot engine with stiff rear suspension. You'll just sit and spin. Each part has to work together, from shocks, spring, tires....etc. The TKO 600 is a tough transmission and I will not tell you that you can't break it. As with any manual transmission, a shock load to the gear set that is closest to the 1 to 1 diametral pitch can and will wipe a gear in any transmission. What is your intent of your car? Give us some details.
I personally like the overdrive, it's the best thing I've ever done. Your options are increasing however, Jamie Passon is working on a 5 speed as well as McLeod. Pricing - I have no idea. Availability - Still a big question but the Mopar options are increasing which is good.
Tony Edler
Hurst Drive Line Conversions
Formerly Classic Mopar 5 Speed