Need A Little help Please

Some rough maths with a 28" tall tyre and 5.13 gears would give you 6160rpm at 100mph over the eighth mile with no converter slip at all. If you estimated say 5 percent slip it would be just under 6500. If you are aiming for more mph it could be up to 7000rpm so i think 5.13 would be good over the eighth mile unless you have a lot slip.