Back to the OP topic. Last I knew the power rating on the 4500/5500 trucks was the same as the 3500/manual. More than enough. Think of it as the "real" power numbers.
For those who haven't run a manual behind the Cummins, it's not at all like what the V8 diesels were like. Which was poor.
While I agree that the latest auto six speeds change the equation for most, fit those who want more overall vehicle control, the man trans hasn't a peer. I especially prefer it when backing and in slow traffic.
I'd recommend a test drive. It's hard to look bad with the auto-throttle. Gives the relations of driver and engine a different feel. Closer. Might be desirable.
Granted I run a 13-speed Cummins at work. Getting into the pickup is like getting into a Corvette. Revs higher and the New Venture has always been smooth.
Clutch on mine is original after 13-years and 215,000-miles. I also read good comments about Valair clutches.