Gear Vendors overdrive

i have one in my car with 355 gears and 235-60-15 tires at 70 mph i'm at 2300 -2400 rpm so far it's been good lots of higway driving . It was in the car when bought. it shifts very quickly and hard . If I were to buy another old 3 speed auto car I would put in a GV overderive unless someone made an 8 speed conversion kit