Are you sure the other end of your pedal support bracket is bolted tight to the dash. If this is not bolted down or loose it will put all the pressure on the firewall.
If the fire wall is flexing you should also see the dash flex .The pedal bracket is attached to the dash and the fire wall and is very solid if it is all bolted tight
Ok so it's been a wile since I installed the booster so I looked this morning and because of the bolt pattern I could not put the two lower bolts in so I guess I'll need to find a way to make a bracket or something