If you have a totally stock E-Series front suspension I would and have ordered the radius arm rear brackets from Ford. You could also find them in a scrap yard but I would go new.
BTW if you go that route consider using urethane bushings---I get mine from Energy Suspension: 4.7107R. If you go that route there is a small caveat about using those parts--you want to reuse or replace the Ford plastic spacer Energy Suspension instructions says to NOT use. That's wrong and they've been told.