Any transmission you get will be a 1:1 top gear ratio unless you get an overdrive. So if you put alot of work into it, unless you put in an overdrive- or make it into a pseudo OD using small rear-end gears, then you'll still be back at square one.
I can say that in terms of automatics, the Powerglide uses the least hp to run, but I don't recommend that for your intentions of driving mostly highway.....or the fun factor.
I don't know what to tell you otherwise.
