Holy converter slip batman

Had exactly the same happen with a friend of mine, '70 challenger, 555hp 440 motor on the day, weight around 3800lbs all in, changed converter from a hughes 3500 stall to a used 10" TCI super street fighter, approx 4000+stall in his car however in the traps he's pulling 6700rpm+ and hitting the limiter and its still pulling he says, 727 was just rebuilt with all new clutches etc.

I worked out that 11.06@121 in 3800lbs with 3.73's/28" slicks/555hp should be around 5427+slip, 6700 is around 20% slip...wtf, got me on that one. If he's blowing through the converter why is it still pulling as he says and it ran its best mph and ET as well but thats cos his ran his best 60 as well.