gear change....possible gain?

thanks guys for the information. Unfortunately the rear end is not a 8.75, it´s a Ford 8.8 so gear change is not this easy....but it seems the only way to find out is to test.

At the moment i´m crossing the stripe around 5900. Don´t know if the converter is slipping....could be. The engine is pulling hard to 6500, so i think the car could benefit from the lower gears. I made some calculations, there is around 6 % speed difference between the old combo 26" / 3.73 and the possible new combo 27" / 4.10, so i´d expect a 6% gain in torque at the starting line - is this correct? I think with the wider tire there´s no problem getting it to hook (car is equipped with SS Springs and drag shocks all around).

Any more info / suggestions welcome!

Michael