Cut it with the bulkhead included. Right where the firewall starts to turn under the car is a massive horizontal bulkhead that helps carry the load from left to right frame rail. Brace it prior to cutting with square tube. Cut from rail to rail above the bulkhead and should be sufficient to hold it square. I would not trust just a strut tower brace to hold square the frame rails if you plan on cutting below the front bulkhead support bar.
It is right above your green highlight in your photo above jutting out from the firewall.