4 speed overdrive or A999 automatic

You've already got the automatic so that will be the cheaper way to go. The OD setup will most likely get better mileage but you'll spend at least an extra $500 rounding up the bell housing, flywheel, clutcth & linkage, etc. to make it work if you have to piece everything together. If you could find a car with all of the clutch & bell housing stuff in a pick-a-part type salvage yard you might be able to get everything for maybe $125 or less. Then you'd just need a new clutch.

If it was me, I'd prefer the OD setup because I like to shift gears.