If the OD transmission has been locked out and is working as a standard 3 speed, why change it? Getting a regular 3 speed has no advantages over what you have unless it is broken. Putting in an SM420 is just added expense considering the cost of the trans, driveshaft mods, and labor with no gain. As one that has probably rebuilt over a hundred OD transmissions, I respectfully disagree with your assessment of the "problems" with them.
Once they are set up and a person learns how to drive a vehicle with one, they are the best kept secret for better drivability on these old trucks. I installed 2 of them last month, one in a 55 150 wagon and the other in a 57 Cameo. The owners love them.

PS. I have several regular 3 speed transmissions.
I can rebuild one and trade you straight across for the OD unit. I can also relieve you of all the cables, solenoids, & relays. You would still have to use your floor shifter or return it to a column shift.
Fred

Last edited by rfs56trk; 07/03/2016 11:32 PM.
1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine