There's some terrific info and photos at
http://www.myalcaponemuseum.com/id27.htm
They post a copy of a calendar from that year and the 14th is marked Valentine's day. There are several links at the top of the webpage with loads of articles and photos on the crime.
As for tying in Cook's death, again I think that line about killing ourselves like all men do is the obvious link. If you're going to persue this idea,(and I think you should)try ending the poem with a third story about how you've killed yourself(figuratively)or probably will kill yourself(literally or figuratively). Use images from the massacre and Cook's death to illustrate the auto-biographical component. That would be a great poem.
In the 30min. or so I spent researching I found out that the massacre ironically contributed to repealing prohibition. So maybe Frank's Valentine to the Irish can be that due to his death, whiskey will be legal again. It might not be easy to pull it off, but it might be worth it.