Automatic or Manual?

[Dust Collector]

I am working on a 71 that I got from my dad. I am converting it from two wheel to four wheel drive (much to his dismay!!). I can't decide what Trans to go with. I have a 4 speed out of a 78 chev with t case as well as a 700r4 and t case. I know the manual is good, it's in my pickup that I am using for a donor. The auto I traded for and "ran when I took it out". Anybody have any thoughts? I will tow stuff with the pickup but most of the time nothing super heavy. I am wanting to get good economy but have heard TBI will gain more than the overdrive in the R4. The 71 is a factory automatic.