Does your car have a race pack or other data reading and storage? If so when does the RPM rate per second start to slow down in 2nd gear?
if it isn't below 7900 RPM you may be short shifting it now.
A engine dyno using a water break is nothing like the motor has to deal with in the car, I learned a long time ago to use the dyno to get the motor close on timing ad jetting and finish the tuning in the car based off of the spark plugs and time slips.
I change the shift RPM by 300 RPM last year in my S/P car from 7000 RPM up to 7300 RPM, the car ran 8.886 at 148.3 MPH having it shift at 7000 RPM, Powerglide 5300 stall nine inch converter with a Biondo 400 box and Biondo air shifter, moving the shift RPM up to 7300 RPM made my car run 8.886 at 150.6 MPH 45 minutes later
I would try shifting yours higher in 200 to 500 RPM increments to find the best shift RPM with your combination