I noticed when I press hard on the brake pedal the clutch pedal slightly moves. Which means the floor pan is flexing. I have a spacer wedged between the master cylinder and steering rack to prevent the master cylinder from moving. Best way to reinforce the area around the pedal cluster?

You sure the pedal box is bolted down tight with all four bolts?

Thats what i would check first

That's not an uncommon area for rust, so check for that, too.

Install Tangerine Racing master cylinder brace, I’m using it for 10+ years, I have lots of confidence when I smash the brake pedal.