With such a high rear end ratio, I wouldn't reccomend anything less than the sm420. The sm420 (used from about 1955-1967) is pretty much the same 4 speed that is currently in your truck, but it is set up for an open driveline. How well is the 283 built? If you have a relatively strong 283 you might be ablt to use a transmission with a higher 1st gear like a muncie or saginaw, but those transmissions would not be a topload design, and you would have to cut a new hole in the floor.
The Sm465 may also work, but the bearing retainer will have to be turned down by a machine shop to fit in your bellhousing.
