You are sure getting a lot of conflicting info. The tranny gurus should chime in here and give you accurate info on the lock-up part, but I didn't think the big blocks got it. But at any rate, you most certainly do not have to have it. A lock-up torque converter is essentially a clutch in the torque converter that engages when cruising which takes out the small percentage of slip inherent in the fluid coupling. That makes for a very efficient connection between the engine and the wheels what increases fuel efficiency and reduces a small amount of heat. Important to a daily driver and for CAFE standards, but not so much for our muscle cars.

It varies somewhat, but converter slip at cruise should be in the single digits. Not a big deal for us. I put an overdrive 518 behind my Hemi, but didn't mess with the lock-up mainly due to the limited selection of converters for my application.

You will have no problem getting a 727 to hold the power you expect to generate. Without too much effort or expense, a 727 can be built to handle nearly twice what you expect to have. They are a great tranny.

As to the converter, that choice should be determined by what you want the car to do and how you intend to drive it. It does sound like you want a fairly stock/original experience with your car. That will be no problem. Just go to one of the many competent converter builders and tell them what you want. They'll build it based on your preferences.