I bought a Duster spoiler from Mopar Performance back in the day. I installed it on my '70 Bee. There was a area where the rock shield had a flat area. I had to put a small notch in it to clear a support.
About that time they determined that one of those spoilers would slow a Duster down in the 1/4!
"The only thing to do for triumph of evil is for good men to do nothing"
