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.