Cory,
26" at 3500 seems a tad low.. Maybe others can chime in on this...
Cam problems can result in low manifold pressure, so the plot
thickens...

Also in case I am misreading, static RPM check R to L is done at full
throttle, not at 1800, or whatever you normally use for mag drop...
And, it will verify that the impulse pin is releasing properly if they
are similar...

Seems like you have a good grasp on engines so I don't want to keep
throwing out picky stuff... But, once you have the cowl off I would
still verify that the butterfly is not loose on the shaft and that it
opens fully...

You don't happen to have a Precise Flight vacuum system, do you?
grasping at straws A fine puzzle we have here... Looks like
measuring the cam is next...

Cheers ... denny