7 " vs 8 " converter?

[danny waters sr]

First of all the car is a 66 chevy ll 4- door 283 220 hp stocker. With driver it weighs 3045. P/G with 202 low gear 28x 9 hoosier ,538 MOSER 12 bolt 4 wheel disc brakes . So far i have had my best 1/4 mile et of 12.25 @ 105.388 mph. 1.65.. 60 ft time ,4.90.. 330 ft ,7.69 1/8 mile @ 88.238 mph. This is with a 7 " converter leaving @ 4200 and shifting @ 6400. any rpm above that just don't pull. It will turn 7000 easy just no power. I had an 8 " in it and ran a 12.42 but could only leave @ 3000 and it was trying to creep then and 60 ft stayed the same as above. I guess what i am asking is would a looser 8 " be faster. It seems like the 7 " don't actually try to lock up til around the 60 ft mark. and is so sloppy loose that i am also having a hard time to find a spot on the tree. Any input would be appreciated. Thanks . If you don't want to say anything on here just pm me. I am happy for the progress i have made in 2 1/2 years as this car started out running 12.98 -13.00's.