I would try a preflash first with non-image light. Basically you want to fog the paper in that area right up to the point where you see the slightest gray, then print your neg. The trick is going to be alignment if you don't use a pin register. What I would do is make a cardboard mask with the mountain cutout. Then use a small penlight to flash the area while slightly moving the cardboard. Even the addition of a little gray should make an improvement.