Looking for overdrive trani advice for /6, please help

I've long wondered about this very "conversation" ( :shaking2: ) .

Seriously , I've thought about the concept of an overdrive behind a Slant for some years . As Dan stated , the A500 (ad seq.) conversion requires a buuuuttload of trans tunnel mods ; then there's the adaptation issue .
It'd probably be relatively easier to slap a 5 speed manual in ; however , you inquired about autos .

How would the final --the direct-- gear be figured ? Don't want to take the engine out of its peak h.p. range , so too low a numerical axle ratio , when combined with an overdriven trans gear , would absolutely kill power and economy . But , at the other end , going to a numerically high ratio may compromise peak torque .

Really , cam specs , and torque converter stall speed are other considerations as well .

Seeing that I don't completely understand all the ins-and-out and upsides-and-downsides , I really couldn't give an honest opinion .
I'm just curious as you about what it'd take to further the potential economy of the Slant , but without compromising power / torque ranges .