i have been there, i had a 95GT with AODE...... with an automatic car with overdrive, the 4.10 is the best gear.
when i got my cobra i went with 4.10's and they were too low for the cobra tranny (3.35 first gear) so i went with 3.73's for a little bit taller first gear.
now as for your speedo, dallas mustang has a SpeedCal that will get your speedo within 1% of accurate. my odometer reads 9.9 miles after traveling 10 miles. SpeedCal is about 90 bux.
i have heard (not confirmed) that the motive 3.90's tend to whine a little. that's why i went with the FRPP 3.73's.
good luck!