if oyu have a complete magnum motor with re-usable pushrods and reusable roller lifters, a regrind is cheaper. even if your stock magnum pushrods are a little short due to the reduced base circle, comp makes some for 5L fords that are .036" longer than stock and work great for $29/set (comp cams PN 7828-16). if you have an LA roller and the pushrods are a little short, Sealed Power PN ZRP3165 are .041" longer than stock, and $1.81 ea ($29 set)
a flat tappet isn't cheaper to retrofit to an '85 up roller motor, if you have the original lifters....for a low-po, old tech cam like the summit cams, at first blush, MAYBE it's cheaper....summit cam and lifter kits are $115 ea, but for a decent quality, higher tech flat tappet cam closer in specs to a roller as far as adv/.050/.2 dur, and lift (i.e. comp XE, lunati voodoo) you're looking at $210 then you're still looking at $90 for pushrods the proper length to retrofit the magnum heads.