Do a search on this forum, as there are several descriptions about how to handle your springs.
Some folks are disassembling their old springs and adding old to new leaf assemblies until they get the ride height correct. It looks like it will take some trial and error to get it right. I have the longbridge BN4 and purchased my new springs from Moss. They aren't installed yet, so I'm not sure where they will land.
One other user here recommended using air lifts to set the height. It would be affordable and fairly simple, but not original.
If you end up too high with the new springs you could add spacers at the ubolts to raise the axle carrier, thus lowering the ride height.
Also, GregW recommended checking the center to center width between spring brackets, 35 1/2". If this is off, it would change your ride height. good luck, Neil