the answer is ... sort of... the early 4.0's will interchange with a 4.2, I believe the 91 to 95 might as well though Ive never tried to interchange between a renix (86-90) or 91-95 with a 4.2 valve cover or vice versa.
The 96 and up is a stamped steel valve cover, as you've found the head is slightly altered so older covers don't fit without grinding that lip off. So that's why it's "sort of" you can easilly grind that lip off and achieve a decent seal using the 96 and later rubber pre-formed gasket. It uses rubber o-ring style reuseable gasket And it's the best valve cover/gasket combo ever made for a AMC straight six IMHO anyway...
The steel helps, but the thick rubber gasket with crush spacers in the cover to keep one from torquing the cover down way too much and distorting the cover is the least leaky of them all (by far) in my experience.
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