The only problem running a A518 is the bellhousing is small block pattern only so it wouldn't work with your 383 without major modification. That being said your cheapest O.D. setup is probably the gear vendors add on O.D. unit.
The cheapest and easiest solution is to just change the rear gears. That's what I did. Went from 4:10 to 3:73 and rpm dropped 400 rpm at 60