The size and shape alone suggest strongly that the part 8 mask was recast directly from a part 6 mask. It also shares certain unusual characteristics of some of the masks to come off the 1986 buck.
Look carefully at the above photo of Gabe Bartalos on the set of Jason Lives with one of his blank pulls. You can see a horizontal "crack" across the mouth holes of the mask that seems to extend from one end to the other. This mask may be a late pull from the movie buck after it cracked under vacuum pressure (as even the most well constructed bucks sometimes do).
The part 8 mask shares a horizontal crack along the same path...
And its also visible all the way across the part 8 "trailer mask":
Though, in case you are wondering, the part 8 masks is much too large to have come off the same buck as the 6. As an interesting aside, the part 7 mask has approximately the same fracture you see in the part 8 mask, visible on the right side.
Which "Jason Lives" mask does the Part 8 descend from? Possibly this one:
As you can see, this old promotional poster features someone wearing a weird mashup of the Part 8 clothes with a leftover Part 6 mask. Courtesty of Mario Kirner, here's a closeup of the same mask from an old video store display:
So we know that Reel EFX or Paramount were still holding on to a 6 mask by the time part 8 was released, and if this was the only one, its the best candidate for the source of the 8.
Looking closely at the scratch patterns around the right side of the mask, you can see a lot of identifiable details line up with the "Alice Cooper mask" used to promote Part 6:
It's almost certainly the same mask. So was the Cooper mask used to create the Part 8 mask? Can't say 100%, but there's a strong possibility that was the case.