Indy/rhs heads?

I'm going to bed, but basically roller cams get the air flow giant lifts do but with less lift duration, rollers can open/close faster/sooner due to the round roller where a flat tappet would eat it's self/cut the lobe off the cam trying the same, and they stay at full lift 'peak flow' longer with less adverse affects to low speed torque and idle, depending on app of corse.

If your head flows 208cfm @.300 & peak flows 245cfm @ .400 lift, the roller cam would be good .420 lift flowing at peak longer getting more of that 245 in.
With a flat tappet you could consider .480'ish lift to lengthen the opening time/flow not worrying ,too an extent, about the stalling at full lift as it's only there for a nano second while the early fill and it's longer duration is more important, don't go too far past peak though.

Hope it wasn't lost in translation.