Exhaust Work for 81 Euro 911

[EuroTargaJim]

I am exempt from emmissions because I rarely drive this car.  I thought "Why not get a killer exhaust and boost horsepower."  I have purchased a pair of Bursch tuned racing headers.  Now I need to install them!  The factory exhaust goes toward the front of the car then bends around and goes into a single inlet cat then muffler.  The headers are designed to go straight out the back.  I need help designing a "cris-cross" muffler system.  I also need the manifold bolts drilled out and re-tapped.  Should the engine be pulled out for this project?  The existing bolt heads are all rusted.  I don't believe I can get them off without destroying them.  The car is a true Euro-Spec.  Am I foolish for attemping this?  Will the car be more valuable left "as-stock"  All you 911 air-cooled experts give me some advice....please!