Let Summit Racing be your friend. Normally on cams they list an RPM range. You obviously want a lower range. Typically these type of cams are referred to as RV cams. Below is a link to a Comp cam part at Summit. Crower makes good cams for AMC straight 6's so I imagine they do good work for Fords as well.
COMP Cams CL66-236-4 - COMP Cams High Energy Cam and Lifter Kits - Overview - SummitRacing.com
Rocker arms won't affect the duration the valve is open, just lift. If you're making a hot rod the larger lift is desirable. I don't know about in a low RPM rig.
Don