Part number is N620484-S56. They are currently on backorder, but there are dealers out there with stock on hand.
If you aren't concerned about originality, most spring suppliers use standard split lock washers and hi-nuts. The hi-nuts give you more thread engagement.
Ford Parts Monkey since 1985
1981 F100 Flareside - Black, 302-4V Roller/AOD
1986 F150 Flareside - Medium Fire Red 302/AOD
1989 F150 Standard Cab 4x4 - Dk Shadow Blue 302/AOD
1993 F350 4x4 Crew Cab - 7.3 IDI/ZF-5
I think it's a sickness...