I have two 27 rear ends and neither one has the double nut arrangement where the radius rods are connected to the u-joint housing. They are not drilled for a cotter pin and just use a nut and lock washer on each one. There is a boss on the rod that goes against the back of the u-joint housing. Is this normal for the 27's? There is really no way to adjust these like on the older cars to center the driveshaft. I assume that just tightening the nuts brings the housing to center or "close enough"
You are correct in your assumption.
The early improved Fords had the two nuts, then sometime during the two year production, it was changed to the one nut type. Since you say your rear ends are 27, they must have been made after the transition to one nut.