1962 235 I-6 vs v8 dilemma

[pestwagon]

Got a 1962 short stepside PU with standard 3 spped column shifted 235. Motor runs (not really well) but it can drive, compression is 125-130 in first 5 cylinders 4- to 60 in 6th hole.

So, it's time to pull the motor. I really want a TH350 or TH400 or TH700 R4 automatic column shifted V8 (like A 350 CU).

I've read many threads on this but still need advise.

1.What steering column years will look "correct" and function to shift the transmissions listed above. I need to buy a automatic column....but I dont want one with a key in it or tilt...I want it to look like a 1962 column....
2. What transmission listed above is recommended? I would like at least 15 mpg on the freeway and am too old to want to race/tear up the truck. I believe the existing tranny crossmember needs to be removed, and a rear tranny mount can be fabbed up with 2 pieces of angle iron on the inside frame rails and a strap of steel across that. Do I mock fit the tranny rear mount first and then bolt a 350 block to the tranny to mock fit the block/motor mounts? What am I looking for here when I mount this? Clearence to the radiator? or clearing the firewall? What have you guys done/can you suggest to mount the motor/tranny in place...
3 Apparently only 283 bellhousing from a 1960-62 pickup will work. What is the reason why? Will a corvette 283 bellhousing work? Will a passenger car bellhousing work?

I am going to start looking for a auto column that'll shift the tranny with the columnn lever, not with a floor shifter. Any advice?

Why isnt there a aftermarket kit for this?