nf fairlane diff into zl? will it fit?

Yes,with alot of work.

Depends whether you want the coil suspension or leaf springs.

zl fairlane has coils from factory

Best fit is the EB1 or EA Diff - as they have the same style of handbrake as your ZL.

EB2 and ED will also fit, but you'll need the EB2 or ED Rear lower control arms as they have larger bushes at the diff side.

EF and EL has been done, but the handbrake is completely different. Doing it nicely is trickier. many people have removed the Floor section from an EF/EL/NL/NF etc and welded it into an earlier vehicle to use the correct handbrake

Nigel