If you run a 40w you won't raise the ambient temp enough, not without making that hot side get well over 100 for a basking spot.

What I would do is put heat tape on the cool side, set it to 82ish on the probe, so that way by the time it gets through the PVC bottom and substrate it'll be in the high 70's. Then you can do a 40w on the warm side to give that basking spot. Your other option is going with an 80w, which will take up nearly half the cage, however this would be your best bet in raising the ambient temp.

80w = 80 watt panel measures approximately 12.5"x22.5"x1.75"

or

100w = 120 watt panel measures approximately 12.5"x32.5"x1.75"