47 Chev--
Gear Vendors products are nice but typically expensive--arguably the only overdrive option for the SM 420.
For an AD truck certainly a cheaper and still effective approach are electronic overdrive transmissions made by Muncie and saginaw. Bolt up to your existing bellhousing--gotta jerry rig a shifter and linkage.
The saginaw is stouter and draws a higher price assuming that the seller knows what he has and the muncie is lighter weight and in my mind fully capable for an AD truck. These were made for GM and other model cars 55--64 or somewhere near that. Typically sell for $50 -- $150 at flea markets, parts sales and on ebay.
The electronics involve some effort but nothin spectacular although some bolters wire them up quite elaborately for gear splittin and the like--overdrive stepdowns are typically 30%--works well with a 3.55 rearend--at least it works fine on my 55.1.
