How important is the plastic belly pan?

Technically air flow, you know how a tech. will answer that! Realistically, don't know if it would make any difference! It is up to you, i have seen them run either way without any trouble, by the way i been doing this longer than most have been alive, for what it's worth.

Its important because it protects the bottom of your car and also keeps the air flows proper inside of the engine compartment. I use zip ties in mine they work just fine. If the bolts snapped of the drill holes next to where the old holes are. If it was a metal skid plate then i would tap new bolts.