Early A, Rack and Pinion conversion, more Qs

the correct method for the rack mounting is to have your innner tie rod pivot point so that your tie rods are parallel to your lower control arms to eliminate bump steer. The idea is that they are on the same plane so there will be no change in steering as your suspension moves in it's travel. As far as the caliper location (front or back) it doesn't matter since pads are not sensitive to front and rear as drum brakes have a leading shoe and a trailing shoe.As for the upper control arms, you should not have to touch them. They are already designed so that you can align your front end correctly (camber, caster and toe).
Going back to your question about front and back alignment of the rack, what you want is the tie rods to be straight with the tires pointing straight ahead. If you were to mount your rack for or aft of center you will experience alot of tire scrub and poor handling.
Don't forget to change your master cylinder after you change to disc brakes as the reservoir for drum brakes is inadequate.
Hope this helps you!