Hi Greg,
Modern Driveline is a well know supplier of T5 upgrades for vintage Mustangs. Several of the Mustang magazines have articles on this very popular swap. Since MD is located close to where I live, the choice was easy for me. My '65 Fastback had 2.78 gears in the back with a T10 as well. Originally running 215/60R14 tires, I was right at 3000 RPM at 75MPH. I have changed to 3.89 gears and the Ford Motorsport T-5z with a 0.63 overdrive. I will run 17" wheels and expect to be at 2500 RPM at 75 MPH in fifth gear.
The owner at MD, Bruce Couture, is a real stand-up guy. I met him through our local Mustang Club. He and his crew have patiently answered my questions and gave me a great tour of his shop outside of Caldwell ID. The website has a lot of info, so even if you select another vendor, there is a lot to learn here. My car is not yet running, but several club members have done this swap using Bruce's services and have nothing but good things to say about it. (And no, I'm not sponsored - though I drive like I am;)
Jim