supergenius,
Welcome aboard, and very nice sleeper I might add.
Nice combo.
The PT6776 turbo utilizes the same compressor wheel that is in the TO4Z turbo. It just has a slightly more efficient turbine wheel. When compared back to back, Precisions PT6776RH out performed the TO4Z dbb with it's unported shroud S cover. If you still have it, what are the numbers engraved on it?
Depending on what type of turbine housing you have, you might want to look into Sound Performances' new quick spool plate that they just released. They have them available for both T3 inlet flange and T4 inlet flange. It was specifically designed to help spool up large turbos on small displacement motors.
http://www.spracingonline.com/projects/
T3 flange pictured at top.
T4 flange pictured at bottom.
This specific valve with any dbb turbo on your combo would be the ticket. Especially if your goals are a reliable 700 to the tires. If I had to pick a turbo with todays Boosted technology for your combo, it would be a PT6765RHP Billet compressor wheel, with that new spool up valve from Sound Performance. The turbo would have either a Precision T3 .63 A/R , or you might be able to get away with a T4 Tangential .68 A/R housing.
Much better turbo than a TO4Z.
Hope some of this helps and welcome aboard.
Patrick