Hey Ole'69...
I am pretty sure you can run hedders on yer 390 w/PS.
My brother, TheeMc, put Hedmans on his '72 CS w/390 and PS...
I have seen several rigs with both...
Maybe it was a particular brand hedders or something.
I know they say that you can't put hedders on a '68 CS either...
But mine had them on it for the past 20 years...
I recently went back to stock manifolds, simply becaseu I had a nearly brand new stock style performance exhaust system on the '71 CS I parted out...
I just did the PS upgrade on my '68 CS... it's well worth the effort.
KaptnKA

S