Polling the group:

What is the recommended or optimum runner length for the primary and
secondary intake paths, optimized for 6000 rpm (2.17 redrive and 2700
prop rpm is the plan, with boost).

The plan is to use a cut-off portion of the stock lower intake from a
2nd gen (mates to the block and changes the intake path 90 degrees).
Attached to this will be intake tubing  (approx 1.5" dia for the
primaries, 1.75" for the secondaries) that will run to a stock throttle
body. The primaries will merge into one just before the stock TB, just
as in the stock manifold. The secondaries will each remain single (again
as in stock).

Dave

You will have trouble mating the tubes to the cut off manifold.
Been there done that. What kind of airplane? How much room
do you have over the engine under the cowl? Are you using the
stock turbo? If not stock then you have more manifold options.

Paul Lamar