I've only had the 0.64 Tremec. The drop in revs from 4th to 5th gear is a lot.
Using the calculator link, you would be doing 72 MPH with the 0.64 at 2K RPM or 57 MPH with the 0.82 at 2K RPM.
When I'm on the highway, I'd like to see the revs at 2K and above, because when the traffic slows down, my RPM's drop down so much that I have to shift into fourth or listen to the engine lug at those low RPM's. And the shift pattern of a Tremec from 4th to 5th (or visa versa) is not the best. One time I downshifted from 5th very fast and hit second instead of 4th.
Can you say Compression Rubber? Yow!
Therefore, I only use 5th on the major highways when I'm doing at least 65 MPH.
With your lower numerical rear end as compared to mine, I would suggest the 0.82 overdrive. You'll be at 2K RPM at 57 MPH and 2500 RPM at 71 MPH. Good RPM's for highway cruising.
Of course, if you live in a state without speed limits and want to cruise at 100 MPH, then go with the 0.64
