Actually what you have is a square bore intake so it is fine with the 4160. No adapter plate needed. The odd thing is I don't recall that OMC ever used a square bore intake on these engines esp that far back since they were using Quadrajets which require a spread bore intake. Is that intake cast iron or aluminum? If aluminum it could be aftermarket (ie Edelbrock, etc). So if the Holley works well on it, leave it be. A bit of a mystery to me though....
Back to your question though, I'd use the open gasket on that.
Since you got my curiosity up, I went back and looked at the BRP catalogs for OMC sterndrives, from 1987-1980 for the 5.7 engines with 4bbl carbs. They ALL used the Quadrajet, with the spreadbore cast iron manifold. So at some point, someone changed the intake on that engine to a square bore to use the Holley 4160. If it works, then no worries. It is very easy to get Holley parts and many marine mechanics are familiar with them. More so than the Quadrajet which has not been made for a long time (since 1990) even though I feel the Quadrajet is a superior carb.
http://shop2.evinrude.com/Index.aspx?s1=csidahqrj56r020jhmkc4tg0s7&catalog_id=0&siteid=1
this is the official OMC catalog, you select sterndrive from the drop down and then find your year, model number etc.