Based on what I have seen from going through a few piles of store backstock, it will be tough to find them by the long box now. Multiple copies in a collection? Definitely. The last long box of ASM 300s that I saw was in 2001 when the owner of a failed LCS blew out the stock in his last storage locker. Unfortunately, they were not bagged/boarded so they all had minor dings.
This book was big back in the day due to 1st Venom appearance and the iconic cover, so the demand was there to drive stores to sell their backstock for a premium from the get go (one of the LCSs dug out a case of NM #98s a couple of years ago, but no ASM #300s since they sold out early). We also did not see the major speculation on McFarlane Spidey until the McSpidey #1s. I was just 14 at the time, but I can remember watching speculators haul out long boxes of each cover at my file LCS since they were going to put their kids through college with them. Based on that, I only bought one copy of each cover for my Spidey collection as even I could see that it would not end well for them as a youth.......