GM made 3 different shackle hole sizes.
1 3/8" i.d.
1 1/2" i.d.
1 3/4" i.d.
1 3/8" is common on the newer '81-on up trucks.
1 1/2" is common on the '80 and older rigs.
1 3/4" is common on heavier 1 tons. I've seen 1 3/8" on K30's, too, so its more a matter of measuring to see what you have.
Go to your local dealership and ask for shackles to fit a '75 truck. It should be 1 1/2" i.d.
I bought two pairs of them brand new and they were like $12 apiece.