Ask them.
My dad ordered a 21in 520 and it didn't fit and got a 23in, no issues. However, you need to discuss this possiblity.
A much better option is to find a shop that has one in stock. There's gotta be a 520 in damn near every size in SoCal. Note that you may have to travel, but if its in stock you won't have to pay retail. I paid $900 for mine new (thats $300 off list) by finding a shop that had had it for a year. They still made a profit as they paid $600 wholesale. If you order, your shop will charge you retail.
Call trek, they probably know who has what.