I had this same debate over a year ago ultimately i went 5 speed with a.64 overdrive. I have 3.91s out back and im running a 28 inch tire putting me at 2100 at highway speed (70mph) with a t56 thats 1600 with 4.10s the 5 speed would put me at 2200 with a t56 speed thats just over 1700. My car runs ok at 1700 but my cam doesnt really start in its curve til around 2200 or so, that being said dropping to an even lower rpm then i am currently would result in a bog feeling on the highway. The t56 is nice i wont deny that however with all the extra fabrication and modifications to suspension components (torsion bars and crossmember) it takes to put one in a classic mopar and as above mentioned depending on cam specs etc i found it to be an easy choice for the 5 speed in my vehicle.