You will need more gear with the TKO 600 @ .64 OD. Even with the 3:55, your final ratio is 2:27. If you are going to go that route, build the engine so your crusing in the sweet spot for your application. I had a 70 Duster running a TKO 600 @ .64 OD with 4:30 gears. All stock except for the transmission and gear; it would run mid 13's (275-60 x 15 BFG Street Drag Radial) and cruise all day long at 75mph (2.75 final ratio)
I think you should go with the low compression build so your combination is in its power band starting around 2200 RPM. With a 3:55 gear you will be crusing at 80mph @ 2200 RPM.
If I had it to do over again, I would not have gone with the overdirve transmission. Not because it wasn't a great setup but I didn't drive it on the highway enough to make it worth the time and money I spent.