No clue I only launch it off a transbrake which'll be totally different than your car rolling in 4th gear. On my two step and transbrake I see 15-16psi at 3700rpm.
My grey car has an identical 67 as well, but no FFI, OS valves or 4" dump pipe thats 18" long. I see the same boost on that by low 4000s. It really wasn't much different than my .70 p-trim 60-1. I think the exhaust wheel has the greatest influence and I've run nothing but P-trims the last 4 years so spoolup has been pretty much the same for me over all that time.