I think you should try and shift it higher. Go up 100rpm each pass and see where it likes it. Also you have a 5000 stall, you are only using 1000rpm if you are shifting it @ 6k. My old 406 with AFR210 made peak power 587hp @ 6700rpm, I shifted it around 7200rpm. Also might want to try a a wilson 4 hole to open spacer. I put one on my 434 and picked up 10hp and cleaned the fuel curve up a ton on the dyno!!! You need to improve your 60' foot. If you cut your 1.5 60' with a 123mph you should be in the 10's.