You won't be able to get the gasket on properly if you don't pull the header out of the way and possibly even the lower splashguard. I just remember how easy it was to put it on when the car was apart for the turbo install compared to when I had to drop the downpipe and struggle with getting the gasket on correctly for 30 minutes.

You will also have to remove the flywheel cover and engine stiffeners to access all the bolts/nuts.