Ok you have two sets of shoes,right.
You put the long shoes on and set the short shoes aside ,right.
I did the same thing no problems,(I can hold 20 lb with foot break,)
You have the S10 wheel cylinder too?
If you use S10 our not the only difference is the braking surface the metal parts are identical.
If you lay the shoes in the drum they should fit nice ,the out side or radius can be a little large to start but should fit. No one radius shoes anymore, I bet no one ever seen it done if under the age of 60.
If the shoes don’t fit in the drum parts store made the mistake.
If the shoes to fit you have something wrong.
PS this mod with shoes and S10 wheel cylinder is for drag racing and driven on the street very carefully!!