Rough-idling 360, can't figure it out!

Make sure you're getting fuel through both sides of your idle circuit. If plugged/restricted on one side, it can cause several cylinders to drop out at idle/low speed and then "come alive" when you get the rpms up enough to start feeding off of the main circuit, similar to what you described. With the Performer manifold, after the engine warms up, you'll feel the two center cylinders on one bank and two outer cylinders on the opposite bank being noticeably colder, as if they're not firing. If it's the centers on the driver's side and outers on the pass side that are cold, your pass side idle circuit is plugged/restricted. The opposite also applies. Either way, a carb teardown/cleaning is in order.