I am thinking about the same things. I think the only real solution is a 4 speed overdrive tranny, as in an A500 (42RH) or A518 (46RG) from the Ram Trucks with the Magnum (1995 or earlier). Either of the transmissions would work, unless you expect to be running high power levels, in which case the A518 would be a better fit (A518 is based on an A727, A500 on an A998/A999). Both trannys bolt up to the LA engine, but the crossmember needs to be modified, and the drive shaft shortened.
It is unfortunate that nobody has come up with a stand alone controller for the 545RFE, which is a 5/6 speed Chrysler tranny used on the newer RAM trucks with the Hemi. Aftermarket can provide a 6 speed (TCI 6X) but that isn't Mopar original, and is very expensive.
3.55 gears with an A500/A518 in overdrive (.69) results in an overall gearing of 2.4495, which should drop RPM and improve gas mileage on the highway, even though the lower gear will allow for terrific pickup around town. The standard torque converters for each are the lockup type.
PATC has an $85 dollar kit that allows the overdrive and torque converter lockup to happen automatically, with a manual override via a toggle switch. Others do as well, and there are articles on DIY setups as well.
Kostas