Typo, I meant if you're using this as a high back booster, not backless. Yes position 6. It clears the headrest but the car seat can't be adjusted to the next level because of the roof.
The 4 in 1 we have (Graco 4Ever DLX) fits the same way. If you're using that seat as a backless booster eventually, you'll likely run into the same problem. The back cannot be raised anymore because of the roof line. As long as our children don't grow more, we'll be ok :p
The gap is only a problem with high back boosters from what I've been reading. The 4 in 1 seat we have doesn't have the same issue.
Problem with the 4 in 1 is that the base is taller, so we can't raise the backrest anymore or it will hit the roof.
For those with kids in high back booster seats, what are you using? The booster seat we purchased doesn't fit properly. The fixed headrests in the backseat cause a gap between the booster seat and the actual car seat which prevents from sitting flush.
Picture above for reference.