Dan said, lots of $. Today, the cam makers offer special drop-in hydro roller lifters to match a steel roller cam. Last I looked, they were over $400 for a set of lifters. I wouldn't worry too much about using a flat tappet lifter in a mild cammed engine. Just be sure to re-lube the lifter faces with a good moly/ZDDP cam lube (tear off the intake on any engine that has sat for a long time, add one of the heavily ZDDP'ed engine oils (Shell Rotella, etc.), some G.M. Engine Oil Supplement. Be sure it starts up right away (timing, gas) and runs at a high rpm level for a good 20 minutes or so.