The '71 in the Ford Dealership is a Mach 1, not a Boss. I wish GT5 had the ol' Boss cars in it.
I was thinking that maybe one of the DLC pastel colors would work, but they are too light. The Fluorescence 001 might be a close match.
Just looked at my paint chips, and these might work as well:
Bleu Non Metallise
Bleu Metallise
Mariner Blue
Caribbean Blue
Vivid Blue Pearl
Bay Side Blue (M)
Fountain Blue (PM)
It's too late for me to try right now, but I'll come back to this thread and run through the above list of colors to see if any of 'em come close. The Boss 429 and the '69 Mustangs have always been some of my favorites as well.
EDIT: I was curious, so I just did a 70 Cammer in DLC Fluorescence 001. I also put chrome Torque Thrust wheels on it, and then I went and bought a Grabber Blue '71 Mach 1 so they could sit side by side in the garage. The DLC painted car is a little lighter, but it's not that bad, I think. I took a picture of it, and I'll see if I can throw it up here to show you. I didn't want to waste the DLC chip just yet, so I didn't save it to my garage.