Though I don't have any personl experience with the tri-powers, I've heard a single 4 barrel will outperform the 3 dueces hands doen and they are MUCH easier to tune that 3 carbs at a time!

For a street car the performer RPM and a Q-jet or Edelbrock would do good for what you're looking for.

6X heads are still easy to come by. I see them on e-bay all the time. Someone here in Houston on the Py board said he sees them all the time in bone yards. For a 400 or 428 you will want the 6X-4 heads. When it comes time to find them you will want to know how to identify them. Keep an eye on the thrifty nickel. The show up in there from time to time and those are more regional than the papers.