Jack21,
This is exactly the advice I got from Will and Bruce from Modern Driveline. Their entire business is swapping T-5's and Tremecs into Mustangs. They carry a full line of trannies and conversion kits. They have a 486hp Mustang that has run a z-spec T5 on the track for 6000 miles without incident. Their opinion is that the z-spec T5 is seirously inderrated, and is just the ticket in application like mine. Everything will fit, and it will last. With a Tremec of any variety, I'm looking at replacing a lot, and I'm not sure how it will fit. I am sold on a new Z-spec. Also, they are right here in San Jose.
Check out their site, it provides a thorough history on the T5 and Tremec trannies.
Modern Driveline