this has recently got very easy
i had a similar problem, or so i thought, with the 545re dodge 5 speed trans behind my late model hemi i'm putting in my '24 dodge coupe. then i found out that they make a gps device that you wire to your electric speedometer. the unit costs just over a hundred bucks, and solves a big problem. i'm using a winters championship quick change rear end, so this will be very helpful. the sensor reads the speed of the vehicle, not the speed or pulses of the driveshaft, so no matter what gears you're using, it'll always read right.