Pontiac lifters will clatter after they sit for a long time. It wasn't clattering when you shut it off, or you wouldn't be posting.

You need to seriously wring out the engine--do it like you are on the show pinks, the title for title race (kinda sorta title to title race.) You can run it at highway speeds for many of hunderd miles, and they sometimes still won't clear up.

Again, wring it out a few times. You'll be surprized at how well it will clear up lifters that clatter from sitting. Mine sat for 6 years until recently. In addition to the clatter lifters, it had a horrendous miss: I pussyfooted it around even at higway speeds. For whatever dubmn reason, I replaced the wires. Still had the horrendous miss. Rang it out a few times, and it purred like a kitten.