I have some experience with this. I have a 427 Cobra replica with a Tremec TKOII 5 speed. This is a tough transmission and you won't easily tear it up but it is slow and hard to shift. I put in a different shift mecanism which helped but it's still not great. I don't remember what my overdrive gear is but it's not the real high one, maybe 0.85:1 or something like that. This works very well and reduces engine speed quite a bit. There is a much higher overdrive gear, ~0.69:1 if I recall. That is too much of a jump when you go from 1:1 to 0.69:1. A lot of people on the Cobra sites reported as a pain in the ass. I guess if you want a quiet engine on the highway that's one thing but it seems like a lot of folks think gas mileage with these high overdrive gears. Why would anyone with cars like ours care about gas mileage is beyond me. I get 8 mph on a good day with the Cobra and I expect worse when the Roadrunner is back in business.
I also considered this for my hemi conversion but there were two reasons I didn't. One, I don't like the feel of the Tremec as much as the Mopar four speed or a Ford toploader for that matter. Two, you have to cut up the trans tunnel and weld in a different one. I didn't want to do that to my car.