I can't answer for '70, but the dozen or so '68-'69 Road Runners I've owned - mostly St Louis, 1 L.A. & 1 Lynch Rd - all had T-posts.
Most were original unmolested cars, a couple that stuff had never moved including the spare tire.
I think my '68-'69 Chargers also had T-posts.
I can recall a tube-type in one of my Dusters, either the '72 or '74.