Fastest/ most powerful slant six?

Back in the 50's and 60's the Ramchargers went through the calculations and actually made one of the first, if not THE first Tunnelram style manifold. The Ramchargers were Chrysler engineers by day and actually calculated things before turning a wrench. Most of the competition were shade tree mechanics. Intake tuning is based on the distance between the valve seat and the carb flange, or air intake. Google Ramchargers High and Mighty Plymouth.

If you use port injection you can easily make all the runners the same length with a common plenum custom manifold for your slanty. Street car or race car? Longer for better torque down low and shorter for higher rpm HP. Compare the early 60's Chrysler 300's for long runner torque monsters to the Dodge 413 and 426 Ramcharger for the higher rpm version.

IMO, the Ramchargers don't get enough credit for what they accomplished. Most of the Mopar Performance parts and manuals came from their research. "We Were the Ramchargers" is a great book to read for any Mopar guy. They gave the actual intake lengths in the book but I don't have it here now.