Looking at that scope, I'd say highs are gonna be needed for the large power ring unless you trim the hammer a bit. You may be able to position the scope so the hammer is between the power ring and ocular bell or just ahead of the power ring and get away with mediums, IF the scope is long enough. There are several scope bases available that will work, the H&R is the most popular, see all of them in the FAQs and Help sticky.
Tim