Looking for overdrive trani advice for /6, please help

There's no overdrive automatic trans that will bolt directly up to a slant-6. You can use pre-'68 truck/van/taxi parts (adaptor ring, flex plate spacer) to bolt up an A500/A518 4-speed overdrive automatic from a Dodge truck or van equipped with 3.9 V6, 5.2 or 5.9 V8, and you can build a control setup for the overdrive and torque converter lockup as shown here, but this will not address the extensive floor pan and crossmember modifications that will be needed. Even bolting one of these transmissions to a V8 (no spacer required) requires floor pan and crossmember mods, much more so for you since you're moving the transmission rearwards.

I think your money, time, and effort would be better spent putting together a good fuel injection and ignition management system rather than doing a transmission swap of this nature.