What a coincidence...I'm also looking for a new valve cover. Do the 4.0 and 4.2 covers swap with each other? I find nothing but plastic ones in the junkyards and the last time I tried to undo an aluminum cover, I snapped it. I've been looking at the cheap Spectre and Trans-Dapt chrome covers on Summit Racing but they don't say they'll replace a factory plastic cover. Am I missing something or is this just the classic case of making every AMC restoration into an I-know-nothing chore?
Your best bet is to get the aftermarket Aluminum, for the cost of the Good plastic cover gasket ($60 last I looked) you're almost there. Just be very careful with the taps.
There are three valve cover patters common on AMCs, and the Eagle used all three. 80 and early 81 were steel; 81-87 were plastic; Beginning mid-87 were Aluminum and were continued unchanged on the 4.0 head.
They all mount to the head in different ways with unique bolt patterns, and are not easily exchanged.
My original engine had an aluminum cover. If you're ever removing the Aluminum cover there is a bolt on the back at the firewall that's darn near impossible to get to, weather aftermarket or stock 87+. Also, the gasket is sealed from the factory with goop, bring a utility knife and a putty knife.