You can mate either an aluminum Powerglide or Turbo 400 to your V6 - either was offered as a factory option in the mid-sixies. You will need the cast adapter housing that bolts to the engine block and then the proper flywheel/flex plate/torque convertor of the transmission you choose to use - again either the Powerglide or Turbo 400. My 1965 Surburan with a 351M has a powerglide behind it - I'm happy with the Powerglide do to the simplicity of the linkage and the amount of abuse they will take. To be honest, if you could get an adapter custom made for a newer transmission with overdrive that would be way better than the old school transmission mentioned - but the linkages and stand alone computer connection may be a nightmare.
