Converting manual to automatic, best transmission to use?

[67ChevNut]

Need recomendations on what type of automatic transmission to use for my 72. It has a 350 with a 4 spd. manual transmission on the floor. This truck was originally manual, I believe it has high gears on the rearend. It feels heavy and slow on the street when driving on 4th gear.

I really don't want to swap the rearend since it is a posi rearend.

I have access to a 2004 4L80 and a 2002 4L60 transmissions, but those are too new for the old 350 in my truck, don't really know how to make them work with it. I also have 350 transmission, but because of my high rearend gears, not sure this one would make my truck run smoother. Thinking about using a TH400 transmission since those came on heavier trucks, although I would have to find one of those. Also, a friend wants to trade me a THM200-R4 transmission for my 4L60, wonder if that would be a good choice.

What should I use? Is there another transmission I should consider? I want my truck to run like it has a 3:08 gear rearend even though I am sure it doesn't.