It wasn't just an od swap, but my trail jeep, 78 wagoneer I was getting 16mpg highway after put a 700r4 in With 4.10s on 33s. I thought that was pretty good.
Same deal with my 74 duster, it wasnt just an od swap. I put in a Re ringed mild smogger 400 and a viper t56, ford 8.8 with 3.73s and 25.5" tires I was getting 20 mpg highway. Mpg has dwindled away though after 10,000 miles.the re ring was just to get the car all setup. Part of me thinks 4.10s would make 6th gear a little more useable.
In the end, they both cost me about the same to do. I can't say it was worth it just for mpg but I drove them both farther and more often after the over drive. If you can afford to waste the cash to do it properly, then yeah you should do it.