904 over-revs going into 2nd

Our 69 Barracuda street/bracket car is over revving (shift point 5500, but revs to 6200) when shifting into 2nd gear. We just replaced the old Hurst Pro-Matic shifter with a B & M Quick Silver (a great inprovement) thinking that was the problem. But during last Friday night's bracket race it still over revs going into 2nd. The transmission (904) has one season on it, is set-up with a low first gear and a shift kit. The converter is a Boss Hogg 3000, again one year old. The engine is 10:1 360 MP 509 hydraulic cam (300-350 hp), 8 3/4 Sure Grip with 3.91 and BFG drag radials. Under throttle it will go into gear, but on the return road under no throttle it just revs.
The trans oil is still clear/red and has no smell.
We're perplexed...
By the way we STILL WON Friday night, 4 rounds (chevy, bye, chevy, chevy)