That's the key reason his passing is of interest to comics' fans. It's not really that difficult to find the issue on GCD--the Series Name of the book is All-New Collectors' Edition (#C-56), so if you search for his name that way it won't come up. But if you search for his name under Character, it does.
I got to see that cover many, many times on my Facebook feed yesterday. It's too bad they didn't meet in the Silver Age, so there would've been a second cover for everyone to post.
I read the Superman vs. Muhammad Ali treasury in tribute.
Here is George Carlin on Muhammad Ali.