For skills, how about: Hide 8, Listen 8, Move Silently 8, Spot 8?
Since its Hit Dice fall between the Huge and Gargantuan monstrous spiders, and its spell resistance and Intelligence are offset by its lack of poison, how about right in the middle of those two: CR 6-7?
Since its magic resistance was only 25%, how about CR + 6?
A typical devil spider’s body is 10 feet long. It weighs about 900 pounds.