my thery on one to two passes being at 148 mph and the rest being from 142 to 144 is the clutch is slipping almost all the time.i got two slips with the normal 1/8 mph with 4.5mph higher 1/4 and and 4 mph on the other one. i pick up about 27 mph on the other slips from 1/8 to 1/4 and then it jumps to 32 mph gain from 1/8 to 1/4. my car picks up 25 mph from middle to end. i got a garnet kit to night watch out next time.
it slipped when stock till i put the 6rr springs in. then i got 21 more hp and put 5 more thousand miles on it it is at 10,400 mi now and i feel the clutch pulse when slideing it out from a sign ot light. i bet it is done
what say you guys?

Clutches are cheap. Replace it.

You should be checking the actuator arm between runs. There needs to be some slack down there, or it will slip. The clutch stack will swell with heat, and if there is no "slop" at the arm, it will slip.

Sounds like the trans was OK after all....

I wish they were for me.