Welcome to the Lone Star club. And welcome to the one real shortcoming of the LS cab design that I see: EV incompatibility. Making it work is a REAL hassle. You basically have to take the amp apart -- remove the back panels, chassis, speaker, and reverb tank. Then you'll see some screws inside the front baffle, which attach it to the cab; undo those and then I believe you can take the front grill panel off, and THEN you can remove/replace the screws. At that point you might consider installing beefier screws, not just longer ones, in case you get the hankering to install one of those 95lb EVM12L monsters. The existing gauge might work... but why not be sure.
Alternately you could bite the bullet and get a WGS Blackhawk HP (http://wgs4.com/content/BHHP), which is modeled after the SRO. I use this as my main speaker with my LSC, and it fits the LS cab like any other speaker. Sounds fantastic. I have not done a direct comparison to an actual SRO, but I'd wager it's in the neighborhood at least... and as I said, it's an awesome speaker. I really like the WGS stuff. I've also tried the Reaper HP (love it as a standalone in a closed back or thiele) and the ET65 (nice in conjunction with other speakers, good by itself too).
I have an EVM12L and when I looked at the installation job it required, I said forget it. Might stick it in another cab.