Looking for Overdrive 4spd Aluminum Transmission

Qs for ya, if I was to put my OD unit behind my 383 in my 69 Roadrunner. What would the outcome be?
Loss of power for better highway ?
Or should I stick with cast iron tranz

Lower first gear and a .73 O/D in 4th gear. But you have to flip the first lever from up to down. You might have to fab a 3-4 shift rod.