There is a good write up on how to adjust the clutch pedal. I haven't ever seen a firewall flex personally but that does not mean it cant happen. I recently installed a higher HP rated clutch and pressure plate, and this took quite a bit of pedal adjustment. You may want to start here and see if that solves your problem. I really doubt that the push rod adjuster could walk in and create this, however sounds like the clutch is not fully disengaging with the pedal in and that's the exact same problem I was having. Specialty Z also has a good write up on this as it is a common problem.