I feel so awful about...?
Any classic car fanatic...and good lord a Mustang collector at that...worth his salt, would never hand over the keys to a Shelby (do you know how rare a Shelby GT 500 is?) to ANYONE. I don't care how good of a friend you are. If this really happened, and the car was fully restored, there is no way that you can ever, EVER pay him back. Not even mentioning the thousands upon thousands of dollars that you cost, the time spent restoring these cars is immeasurable. Hopefully your friend will have learned his lesson and has taken your keys away, if not then he got what he deserved. You may not like this answer, but it is what it is. I know I seem mean, and letting you drive was your friends choice, but you really should not have been out in those conditions in such a valuable car that did not belong to you : )
edit: sorry if I was not nice, it is just that there is no way you can possibly pay him back. If he is glad that you are ok and has left it at that then maybe it is best for you to leave it alone...no sense whipping a dead horse. I wouldn't drive any of his cars anymore though.