I have a 65 Barracuda with a slant 6 - three on the tree want some transmission/engine advice

Just know that you will need to cut a large part of the floor and transmission tunnel out and replace it with the four speed floor. You cannot get around it, because the four speed transmission is much wider than the small three speed. I have a 64 Valiant with a hot slant 6 and the same transmission setup. Three on the tree. My wife loves driving it. Teach your wife to drive it and have fun.