Hey that sounds like a fun project! The 5th over drive gear ratio problem still applies. The "stock" or more "typical" OD ratio used today is just to dam high to be worth much. Such a BIG drop in rpm from 4th to 5th, tends to "bog" a high perfromance engine and leads to problems in general.
Building the motor (cam selection mostly) to MATCH your selected cruising/performance gearing (rear/trans) is a BIG factor in getting the car "right".
Choose wisely.
