I think you are confusing roller rocker arms with roller camshafts.
They are two different things. The E7TE heads have bolt-down pedestal mount rocker arms. You an buy pedestal mount bolt-on roller rockers. They just bolt right on, though you need to check clearance with the valve cover and it is a good idea to know how to check lifter preload too.
When you say "flat tappet" that is a camshaft lifter term. Mustangs that came with EZTE heads had hydraulic roller lifters, not flat tappet lifters.