Embarrassment for your time slip wont help you find your problems. Its the simplest form of data logging out there.
Just because it was rebuilt doesnt mean it was rebuilt correctly, down to the last bolt.
How many passes did you make at the track? Was the time consistently slow? What transmission are you running? Did the clutch slip, converter pressure fall?
Get a after market boost gauge to make sure its holding boost at higher RPM.
Tire spin isn't always that noticeable to the driver.
Check all of your IC pipe for leaks, and remember that under pressure everything is different: joints that look good might get blow by under boost. Tighten all clamps.
When you get it back from the shop again, I would still touch everything myself. Its the only way to really know. Comp test and leak down.
Hope it gets worked out for ya.