you are right on the brink
you probably don't need to use a board but make sure construction is extra careful extra exact
an idea would be to use an 8" dummy 1 or 2 inches tall under the very bottom and ice it all as if it were the 8"--then you for sure are good
if you do use a board for the 6 inch tall 8" cake--then you've doubled your servings for that tier--i'd place the board halfway up