Dan there are a couple of approaches to put disks on your existing straight axle. One switches you to 5-lug hubs and the other stays 6-lug. There are pros and cons, but I think deciding if you want to run 5- or 6-lug is the first decision to make before deciding the rest of it.
If your 283 mounts off the front of the motor any small block with the holes in the front of the block will swap in its place. There are also kits to install a motor with sidemounts instead of the front mount.
Did the truck have a manual or automatic transmission? If a manual most any later GM manual can be put in its place with a little work. If you want to run an automatic you probably want to sidemount the motor and run a more recent transmission.
My '55 1st Suburban is running 5-lug front disks. Someday we'll arrange a meet and you can look that over. I'm still running an inline and stock 4-speed, but have replaced the rear axle with a later run improving my crusing speed and gas milage.
283s are great motors. I'd sure be tempted to rebuild it especially if it is actually the original motor. If you replace it with a later small block keep all the original stuff. Someone will want it.
