If I am understanding your set-up correctly, it sounds very similiar to one that I used to have.

So are you saying that the temp on top of the plexiglass (inside the cage) is about 90 degrees, but on top of the substrate it is only 77? If so...then that is right. If the snake can get to the 90 degree plexiglass then you don't want it any hotter. If he is cold, he will burrow down in the substrate and get warmer.

As far as the ambient temps, that does sound a little low. In my cage I put a radiant heat panel from Reptile Basics on the ceiling of my cool side. That boosted my ambient to 80, so I had a perfect gradient from 80 to 90 degrees.

Hopefully some of this helps...