Any recommendation for a shifter for a 66 with bench seat and Muncie M-20?

I found a factory 67 Chevelle 4speed shifter for bench seat or would you go aftermarket?

hurst without a doubt

I agree with Adman... I'd go with a Hurst, it will shift much nicer, and probably cost the same or less than a repro or nice original stock shifter.

If you want it to "look" stock, then you could make an adapter to mount the stock Chevelle handle...