We use Pioneer OS-938. Viton "positive" seal. Same guide boss machining mousefink speaks of.
When the guide is replaced, often what's left of the original (a "shell" of iron about .020" thick) cracks or breaks off. This leaves no material for the original .535" size. For this, a .500" seal is called for. We use SBI 122-1275. It's also Viton, designed for the '82-'83 Ford 3.8 V6. Nice fit. Put them on all 16 valves. Delete the steel "splash shields" (dead weight).
We have also seen a significant number of "lined" guides fail due to the liner moving (in several engine families, including Pontiac). We don't use the process. We install true "thin-wall" guides (.502" OD). Avoid "spiraled" guides. For anything above .520" lift, bronze is called for (resists "side-loading" better than iron). There are some "hardened iron" guides available too, but we prefer the bronze. For under .520" lift, a stock replacement is fine.
While you're "in there" is the time to install screw-in rocker studs, too. It takes less work for the Pontiac than a Ford or Chevy small block. After the original studs are removed, the holes are perfect for 7/16-14 HeliCoils. No other machining is required, and you can utilize the stock pushrods, guideplates and bottleneck studs for a "non-adjustable" valve train if the cam has a stock base circle. HeliCoils are stronger than iron threads, so relax about that. It must be done on a precision machine, though (no hand-tapping).