CR 7 works for me. At that level, Small, Medium and Large monstrous spiders will all still give XP to those fighting them.
Ooh goody, another spider! I love spider monsters.
I agree with aiming for a Challenge Rating of 6-7.
Right. Since they're 9 HD shall we make them Huge Magical Beasts, since a phase spider is a 5HD Large Magical Beast and a Large Monstrous Spider is a 4 HD Vermin? That also fits its 2d8 bite damage, since a Huge Monstrous Spider has a 2d6 bite, which is pretty close.
Ability score wise, here's a Huge Phase Spider and a Huge Monstrous Spider for comparison:
Huge Phase Spider: Str 25, Dex 15, Con 20, Int 7, Wis 13, Cha 10
Huge Monstrous Spider: Str 19, Dex 17, Con 14, Int 0, Wis 10, Cha 2
The phase spider seems the most suitable model, although since we're modelling these as spider "royalty" which command other spiders I'm thinking they should have a higher Charisma and lower Constitution. Since they have good AC for a spider, I'd give them the Monstrous Spider's Dexterity. Maybe something like:
Spider Queen: Str 24, Dex 17, Con 18, Int 7, Wis 13, Cha 16
Since the original's got AC1, that works out as AC19 in 3E, so I'm thinking Dex 17 and +8 natural armour.