3sp to m420 conversion

[Mr Big]

So I've been using my old 66 for 20yrs hauling whatever I could get in the bed...from 30+ bales of hay, 20 railroad timbers, to a ton plus of creek gravel. I've now messed up a second 3sp. I could repair but was thinking of converting to an AS BUILT 4sp. Its my understanding I would need a tranny and the floor and firewall humps, maybe a floor shift column to keep it looking factory. This truck is stock/ original front to back the 283 has never been out of the frame. The truck has received regular service.

Anyone have a parts truck near TN?

Thanks for looking. Glen