[Longhorn Man]

very easy swap, and could have been ordered stock this way... but not many were.
A TH350 would be the easy way, common, and plenty strong for any use you'd put it to...
Parts list;
Tranny
Converter
Converter bolts
Flex plate
Tranny cooler lines
Tranny cooler, or a rad with built in cooler.
Shifter (recomend aftermarket kind, stock style can be a pain in the rear toset up/locate all parts)
Automatic tranny cross member
Tranny mount
Possibly need a speedo cable. (length issue)
Starter
Most, if not all of this can be obtained at a junk yard from any automatic equipped truck. (67 - 72)

Drop the old tranny and bellhousing, pull off the flywheel, drill out rivits holding tranny mout arms from the frame and throw into the scrap pile.
Measure the two trannys, if the length is not the same (give or take 1/4 inch) then send out the driveshaft for mods.
install flex plate
install tranny
install mount on tranny if not done already
install cross member
install driveshaft
run cooler lines
install new rad or very big stand alone cooler
install shifter
lotsa tranny fluit (replace the filter too if you are installing a used tranny)
Install speedo cable

Tail lights and tire smoke... have fun.