A reach doesn't last without adding an additional leaf for support
I've had several sets rebuilt and done this way.
On my 78 fury 2 door, I pulled them, had them reached and it didn't last a season
They were sagged as bad (or worse) than they started out with. I ended up band aiding it for a few cruising seasons with air shocks and last year I finally ordered a set of new ones from general KC spring.
In 78 that car could have come with a 4 leaf, a 5 leaf or a 6 leaf spring. Mine was originally a 4 leaf pack, I went with a 5 leaf and like how it rides now. I went back with standard shocks and got rid of the air shocks.
I've had mixed results with adding a leaf. On my diplomats an extra leaf improved the ride, as it did on my 94 Dakota (reg cab/short bed) but on my 96 Dakota (4wd, cab and 1/2) and especially on my 01 Durango an extra leaf made those like lumber wagons. The Durango though was all over the road, wishy washy no matter what else I'd done prior to the new springs, and the new springs cured that.