The stock seals are the O-ring type valve stem seals that go in the second groove on the valve stem. In addition to the stem seals, Pontiac always used a steel spring splash shield over the valve springs to reduce direct oil splash onto the valve stems: A lot of people delete these splash shields during assembly, and see increased oil consumption when using only the upper o-ring stem seals.
The "umbrella" seals that come in the kits are pretty cheap and cheesy - don't use those. Rather, if you want to go with a "PC" (Positive Control) oil seal, use one of the good sets of PC seals with the metal compression bands. Yes, the valve guides need to be machined to accept the PC seals - a common operation for any good machine shop.