Here's an example of using conveyer material to make wheel stakes. It was done for someone with a pickup truck. On your Mustang, they would be tire width, 3-4" deep and as long as you care to go. These, ahead of and behind the wheels and with a full bellypan would really clean up the underside.
Here it is applied to your situation (roughly):
The belt would be rivetted on the two sides but not across the width, so the belt material can flex. The free edge next to the tire would probably assume a curve shape. Go long and let them wear off on the bottom