Do the bolt hole pattern differ slightly slightly as well as the ID of the pipes ?

I believe that is the case. I think the flange may even be a different size overall.

--DD

If you are talking about the Bursch flanges Jim, then yes.

Jim, the he muffler flange bolt hole dimentions differ 1.7/1.8 vs 2.0 versions.

Yup: just confirmed with Chris st Tangerine. Thanks.

I had thought my HEs were 2 L for some reason until Bruce clued me in on how to tell the difference. The 1.8s kick upward at the back...