It seems to me that at one time, the rear spoilers (deck lid and quarter extensions) were available from a fiberglass supplier to make a cheap SHELBY look-a-like. Don't forget that the 1968 MUSTANG C/S was a HDTP and those spoilers (if available separately) might be easier to fit on the COUGAR.
You already have the correct rear body panel(the SHELBY MUSTANG used a fiberglass panel over the origional steel panel to accept the COUGAR rear lamps. So you have the correct lamps and all you would need is the SHELBY lamp doors to finish of that detail. You would have to work the rear body panel to accept the MUST fuel filler and use the pop open cap.
The reveal mouldings on the deck lid edge might have to go. Rear face bar is good and you might have to tweak the lower valance for tag mount and for duals. Maybe mount a set of SHELBY lower scoops to the quarters. Of course that padded top would have to go.
I think it would make a nice looking conversion if done tastefully. Something to think about.
I am looking for information concerning factory performance/aftermarket speed parts (1958/1960) used on the MEL (Mercury-Edsel-Lincoln) (Also Thunderbird 59/60) Engine Series (383-410-430-462) produced from 1958 thru 1968.
Also older FORD Special Service Tools
-MEL Engine Forum-