Why a 6 speed automatic transmission?

The 70mph=2000 rpms has more to do with the rear gear (differential) gear ratio than the transmission. If you to want to get the rpms into the 1600 range then ask or look at a vehicles with different gear ratios in the differential. If you were driving cars with say 3 : 50 rear gears, see if they have or look for one with 3:20's or so