I'm nowhere near an alignment expert but from what I've learned around here stock alignment specs seem to be the root of all unhappiness.  Especially the stock caster settings.  Others will be along with more knowledgable suggestions or to call me a fool .

Like MS says, do your self a favor and change everything from the steering box, including the pitman arm, out to the spindles then get your alignment. Everything is available new from most Mustang suppliers. The V8 parts are very different from the 6 cylinder parts. I changed everything when I installed V8 spindles on my T code.

Typically if the car has not been lowered, the caster is set to zero, but I always like to add a couple degrees positive caster.  It helps the car go straighter down the road.  Set camber at zero.  That puts the wheel straight up and down.  Toe-in is 1/8", but most of the computer-whiz-bang-don't-really-know-what-is-going-on youngsters measure it in degrees because that is what their machine does.  Try and find one that can explain what he is actually doing...

I do not think the control valve is removable on the 65/66 cars.