A frame is a frame is a frame, right?
 

84 F150 4x4 frame is the same as an 86 two wheel drive, correct?

The 86 is "mint" vs my 84 with rear shock towers rusted off, and thin around the spring horns.
I understand the front suspension is completely different.
But finding another short bed is unusual (around here)

If we're defining a "short bed" as 116", the Parts book sez they're both E2TZ-5005-M:

http://imageshack.com/a/img138/434/7kpm.jpg
http://imageshack.com/a/img199/5689/7qaz.jpg

The rear springs are wider on the 4x4 frames, so the brackets for the springs are wider on the 4x4 frame. You would have to swap the brackets(which you said were rusted) to use the wider 4x4 springs. You could just use the narrower 2wd rear springs, but if you want to get technical, that's the difference.

There is less difference in the 2wd vs 4wd frame up front. In fact there is no difference. If you unbolt the suspensions from both trucks, they are then the same, and will swap easily from frame to frame.

Yeh, I figure I must get the spring mounts, so I'll get the 4x4 ones

That is the great thing about the Twin Traction Beam 4WD setup. It is so easy to switch between the twin I beam 2WD and the TTB. I'd just use the 4WD leafs in the back. Less work, and it will give you better springs on the 2WD.

Get the spacer that is used on the 4wd trucks between the rear springs and the axle.