should I change gears or tranny???

The gears would be a cheaper route to take.

The O.D. trans would be really nice on a few levels except price and labor. You would;
Maintain the current get up and go abilty while increaseing the mileage abilty with the .69 O.D. ratio. 4.10 X's .69 = 282.9.

The earliest 518 trans you can find with the manual switch to turn off the O.D. gear would be best. The actual number/letters of the trans escapes me right now.