GT options

If you're paying up for a near #1 car part of that value will be in authenticity. If it wasn't a GT you probably don't want to convert as you will be drilling some holes and changing a few parts. I have a non-GT car and find it is often the least common car at shows as there are so many fake GTs out there. (Not meaning to knock your idea.)

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Will I hurt the value of the car by adding the GT options, such as: rally-pac...

The Rally-Pac was not a part of the GT option. Any Mustang could be ordered with one.

If you don't plan to sell your car at Auction or Show it in a points class ANY bolt on modification is OK - in my opinion - and while it is YOUR car - enjoy it the way you want.

The Rally Pac and most of the GT items could be added as dealer options anyway. About the only thing that requires "HOLES" is the GT fog light wiring (2 or 3). AND these can be easily hidden or repaired with a little work. If you like the GT exhaust - get a repop rear valance - have it painted to match the car and swap out the original one - you can always change it back later if you want.

While I agree a non GT HiPo is a rarity at shows - real HiPos or cars with intact drivelines are just as rare. I see 100 to 1 cars with HiPo badges verses real HiPos at shows - and this number continues to go up every year.

I have a non GT '65 2+2 HiPo - with the original drive line intact, but I am adding the GT fog lights and exhaust trumpets to mine - because I like the way they look, I don't plan to ever sell the car, show it much, and I don't plan to ever pass my car off as a factory GT - even though I could based upon my build date. Others would not be as honest. Plus I have pictures of the car back to '79 when I purchased it showing the non GT status - these photos add value to the car in my opinion because I can tell anyone the history of the car and exactly what has been done to it for over 75% of it's life. My kids can argue over who gets it or sells it when I am gone!

This is not too different from adding Disc Brakes to a non GT car that came with drum brakes or adding a dual master cylinder for safety reasons. While non-stock they can be "undone" with a little work.

You didn't mention the rocker panel stripes and "MUSTANG GT" badges. It sounds like you just want to add some cool over the counter options/additions to your stang. Go for it. It's not like you are trying to present it as a GT.