I decided to change the interior color in my 74 a few years after having a new windshield installed. I didn't want to risk breaking the new windshield by taking it back out either.
The hardest area to paint was the 1/2" or so wide strip right up next to the windshield. The angle of the 68-79 dashes in that area makes it impossible to spray that section from the inside with the windshield in place. And leaving that area unpainted would be very noticible when looking in from the outside (especially on a color change). I taped some fine sandpaper to a paint stir stick to sand that area. And bent the end of a small paint brush (the type typically used to paint models) so it would reach down into that area. I placed a small mirror outside the windshield and used that to watch where I was painting. It was quite a tedious PITA process though.
With that area painted, I used some posterboard (thin cardboard) to mask off the inside of the windshield. I trimmed the lower edge to fit, slid it down into place, and taped it at the top. Unlike paper, the posterboard was stiff enough to stay up against the glass and not sag down onto the dash. Then I sprayed the rest of the upper dash. The area right up next to the glass that I had previously brushed received enough overspray to blend it in and hide any brush streaks. I did get a tiny bit of overspray on the edge of the urethane windshield seal but its not that noticible.