As far as regular 58-59 1/2 T pickups go, the short fleet is the most valuable in most markets. 6 CYL usually not worth as much as V8. I have not seen any 6 CYL with automatic.
BTW: Rare does not always mean valuable.
Many folks like automatics from those years. Many people like stick shift. Not much about swaps is straight forward. Rigging up the clutch, pedal, bell housing, drive shaft...is all work. Can be a "labor of love" also. If you find a 4 speed from a 58-59 donor truck, it will probably be a bolt on situation. Try to get all the parts that I mentioned above from one short bed truck. 55-59 donor will probably work. If the truck is rust free and original, including drive train, I would not touch it, it should sell back east for good money. Actual money value is impossible to say without looking at it, driving it and knowing the market in your area of the U.S.. You probably know what it's worth because you bought it.
