A few years ago there was a convertible in Florida that had been changed to red. A friend of mine had a 4speed when we were in high school. Other than that, I have never seen one.
GTX comes with a hemi standard. Roadrunner does not. Actually the standard engine for the GTX was the 440-4bbl, the 426 HEMI was an option, throughout it's entire production run as a stand-alone model from 67-71'. The 440 6bbl was also available as an option in 70-71'. The GTX was considered the "Gentleman's muscle car" as it had a higher trim level and price tag. The Road Runner in those years was initially a bare-bones/big motor version of the GTX, 383HP being the standard engine in 68', the 426 Hemi and, from 69 1/2', the 440 6bbl were optional up until 71'. It was marketed toward a younger crowd. For 72-74', with the GTX discontinued as a stand-model, Road Runners ordered with the optional 440HP engine were called a "Road Runner GTX".
There are many variables incuding camshaft selection, headers, is it carbureted or fuel injected? Turbo (s)? Nitrous? Etc., etc. Normally a good rule of thumb for a mild rebuild with 4 barrel and headers is approximately .75 to 1.25 horsepower per cubic inch. Example: 302ci @ .030 over is 306ci. 306ci x .75hp = 229.5hp 306ci x 1.25hp=383hp With an average 306hp It will move out pretty quick in a light car like a Mustang. These are just averages and could range greatly one way or the other depending on how radical or mild you decide to build it. With a single turbo and some good tuning 475-500hp isn't out of the question. At that point you will want to seriously consider strengthening the bottom end with a forged crank. I could ramble but it will get edited out anyway Good Luck with the build!