After swapping my LB9 305 for a 383 stroker, I left the SLP Cold Air box #21010 with K&N filter #E-0890. That was a temporary solution. Datalogging showed almost 2inHg of vacuum at WOT in the higher RPM. That was a bit more than the 305. I did some testing for kicks. Here are the results:

Constant atmospheric pressure at around 102.2kpa (30.2inHg). I am right at sea level.
Intake Air Charge (IAC) equal to outside air temperature, so cold air intake working

Baseline - SLP Cold Air Intake #21010
Line 1 (grey) - No air intake at all, just like on engine dyno
Line 2 (blue) - SLP Cold Air Intake #21010, no air box, no filter
Line 3 (orange) - SLP Cold Air Intake #21010,no filter
Line 5 (Green) - Spectre Performance Custom 4in Cold Air Intake

Line 1 and 5 are now equal. I can conclude that the air intake pre throttle is not creating any restriction anymore. Basic stuff, but nice to capture on a graph.

Here's a video: