Larry, I am not sure what pinion offset has to do with narrowing a 9" rear end.
I had a '70 9" narrowed by Currie to fit my '65 (plus narrowed an extra 1/2" on each side so 8 1/2" rims would fit). Pinion effects never entered into the conversation.
HistoricMustang
www.historicmustang.com