What is the best way to import this non-standard text file into SQL Server?
Based on your sample data it looks like there are 6 columns in total.

COL1, COL2, COL4, COL5, COL6 = All fixed width .

COL3 = This is a fixed width for a given day, but it is variable day to day (based on my understanding).

You should be able to calculate COL3's width each day based on the length of any given line. You know COL1,2,4,5,6 are all fixed width. Take the line total and subtract COL1,2,4,5,6 to get COL3's width each day.

COL3 = LINE_WIDTH - (COL1 + COL2 + COL4 + COL5 + COL6)


After you get COL3's width you will be able to figure out all of your start and end points.