I use a 246/246 hyd roller in my 512 with a Sniper + HyperSpark combo. My engine has 15 inches of vacuum at 850 rpm with that cam. Part of the trick is the HyperSpark ignition since it allows me to run 25 degrees of advance at idle. So I think your 242/248 should be okay but anything smaller should also work just fine. Here is a link to the Hot Rod article I wrote on my install: https://www.hotrod.com/articles/holley-hyperspark-install-tune/