Getting ready to start assembly of new bed. Have printed out most comments about this and have books but still have some questions.

Have 3 tall cross sills and 2 short ones plus rear cross sill. obvious where tall and rear sills go, but what about the short ones.

my frame does not have holes thru bottom flange in any locations where tall cross sills are mounted. will i have to drill, or can i just mount to top flange of frame where hole already is.? I refurbished cross sills from a 54 long bed, but all other parts are new from Chevy Duty.