I Buck,
You might have cut it too short, as that will pull the rest fo the system to the rear and try to pluu the down pipe "up". Loosen it up and see if it falls off the firewall. Also remember when you tighten the flange at the turbo you have some adjustment there to go up down etc too. The firewall cover is pretty tough stuff, shouldn't bother it anyway but check out the fit and see if it will drop off of it.
Mark @ DPPI