The Spoiler is 57.5" from tip to tip. The mounting brackets are 18" out from the center on both sides. Don't know what you plan on mounting it on but it doesn't sound like a 69' fastback. I have a 67' coupe and installed one on it, here's a couple pictures. It fit like a glove, the down tips match great with the quarter fenders. If you are putting it on an older coupe, make sure you don't place it too far back or the pedestals don't lay flat on the trunk.
P.S. By the way that was the size for a 69-70 spoiler, the 71+ spoilers are different. I don't have that so I can't help you there