Mark I and Mark II are identical in terms of engine bays... you have a picture of the header put of the car? Was it an eBay purchase?
Chances are, what you see I had to do, you will, too... because it's usually the downpipe portion that doesn't clear the lower firewall and floorboards. Just make sure you have some biiiiiig Jack stands for both height reasons (so you can get enough swing with the hammer) and for stability (because the bigger truck Jack stands have sturdy, wide bottoms).
Chalk it out (or a grease pencil, whatever), and get to hammering. These things never fit our cars without modification. The good news is doing this will leave your exhaust tucked up high behind the frame rails so if you have your car low like mine, you don't have to worry about crushing or even scraping the exhaust