Are you replacing the faucets just because you want new ones? Or because you think you must for the PEX?
If your '71 is set up like my '75 rear bath, replacing the shower valves will be a big project. Then the PO of my trailer put in PEX, he left the copper running from the shower into the vanity, and tied into the PEX there.
Il Carriaggio — 1975 Argosy 24 | Il Progetto — 1976 Argosy 28 Center Bath | WBCCI# 15566
He has all of the virtues I dislike and none of the vices I admire. — Sir Winston Churchill