I (used to) flight this route fairly often. Sometimes through SEA and sometimes through SLC when that was available. It is a very elite heavy route as you'll see the same people in F week after a week. The only times this changes is on special events such as the rodeo (either in LAS or YYC) or some other conference where there are large groups flying from one city to another.

If you can't pick a seat on the DL.com then give them a call. Get it sorted now, otherwise you'll not only not be in F, but you'll be in row 30 in the middle with no recourse.