scott you musta been reading my mind. I was about to post the exact same question. My 66 has the 3 speed/no support in it and I'm going to a significantly larger MY6/89MM 4 speed and wondered if I needed a crossmember.
I think I'm going to be on the safe side and buy one I saw at Golden State parts for 50 bucks. It can only help.
I don't think you can go wrong if you go ahead and get one for your 4 speed. Overkill at best.
-ftyler
