Don, I don't know if 20 degrees is correct or not, instinctively it sounds a bit much, but that's neither here nor there. What I would suggest is you lay the stanchion on a piece of poster board or other similar pattern making material. Trace the outline of the stanchion, and then make several. Now you'll be able to "practice" how much to cut out to get the slant you want. Tape it to the car and stand back so you get a good perspective on it and make sure you like it before commiting to doing it in steel. You could even use the mock up to determine what your net height change will be. Again, not sure it would be an entire inch, but that's what mocking up is for. As for the lug on top for the header, once you've got the lower angle figured, you can just cut the upper pivot hole area loose (a smile cut?) and rotate it slightly forward to get the lug "verticle" again. Need just a small filler piece in the rear (probably use the small piece trimmed off the front if they're the same shape) then and no revision of pivot point orientation. As far back as you would sit from the windshield I can't imagine you'd have any interference with your head in normal use.

There are a number of windshield posts made from cast aluminum that have a slot in them to "drop" in (actually glued in) a piece of windshield glass cut to fit. Although most seem to be for the '32 guys, rather than A's. These parts are aimed more at the smoothie style crowd as it eliminates the frame around the glass and the pivoting feature. Not sure if I've ever seen then with a pronounced slant back, but then I don't tend to be interested in that style.