I would think the misfire would not cause these codes. These codes are for too lean, meaning the 02 sensor sees too much air. I would think a misfire would go the other way.
There are a number of components that can contribute to a lean condition. I have a 99 and I never resolved the issue, even by performing the required bolt fix.
You should have a metal upper intake on your engine, I am not sure I would call it a plenum. Regardless, you should not have the bolt issue on your vehicle. The bolt issue is due to isolators needed to bolt the cheesy plastic intake on the metal lower portion.