According to the master parts book:

Part# 3682788 shackle is used both front and rear on a '56.

Listed for:
rear 55-59, ½, ¾ ton
65-67 ser 20, 30 D-D

Front 47-55 comm
55-59
63-66 4/WD

It lists overall length as 4 1/16" and width 1 ½".

On ADs they are different. I found out after ordering the same ones for front and rear from LMC.

"It's just a phase. He'll grow out of it." Mama, 1964

1956 Chevy 1/2-ton 3100
1953 Chevy 6100 "The Yard dog"
1954 GMC Suburban Now with a new proud owner.