My suggestion on the '40 would be to get the car set and leveled, then install some bracing and cross pieces clamped to the frame rails so that things don't start jumping around when you remove the junk that is in there now!!! You might even want to consider some sort of fixture on the front of the rails to secure them and get everything square and level before the old crossmember is removed and the new installed! Measure, measure, measure!!!! I do see what you mean about the bolt in being a consideration....if there are enough of the original attachment points left on the rails that you will have valid reference points????