There are some weird things that start happening in 77 and 78. Three fresh tests isn't going to do you much good anyway, so I'd want to take those two. Keep 49 if it makes you feel better to have one.
How many PTs are you guys doing before D-Day? I was thinking 3 full-length timed (76, 77 and 78) with one un-timed (75) 2 days before just to reinforce fundamentals while not stressing over timed score?