Rear leaf spring Question 74 Dart

I'm going with 6 leafs on my 75 dart. My dart had a really bad sag to it so I changed the leafs with factory replacement 5 leafs and it didnt help anything the sag was still there. So I added shackles to bring the rear up. (Yes I know it's a horrible thing to do but I was young and didnt know any better back then) so now I'm going with the 6 leafs from general spring.