I found an OEM part from a different vehicle that fits on my '69 GTS like a glove. I bought the part new on-line for only $28 shipped.
Here it is test fitted on my car. There isn't any mounting hardware attached yet but you can see from the pics how easy it will be to get it fully mounted.
I'm sure that if it fits on my GTS it will fit on many other A bodies too.
It is the OEM air dam from a 1991 Ford Ranger Compact pick-up truck. The actual part is only $21 plus shipping. Here is a link to get one...
ReplaceĀ® FO1092161 - Front Bumper Valance