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On the OBS trucks there were pre-requisite options the truck must have in order fot he z-71 option to be placed on it.
The truck must be :
4x4
Silverado/SLE trim level or better
265/75/r16 tires
alloy wheels
FWIW Every truck I have seen that is a real Z-71 has had the 5.7L. or the 6.5L Some people on the board say that they have seen some with the 4.3L V6. I personally have never seen this (and I deal with a lot of trucks)
Z-71 was available with any cab or bed configuration available on K1500
Available with automatic or 5 speed up to 95
96-up I think only available as a automatic since the only trans option for the 5.7L Vortec was the 4L60e. You may have been able to get the NV4500 with a 6.5L I don't know.
OBS the Z-71 option added:
Heavy duty Bilstein shocks
Skid plates
bed decals
The other things mentioned by posters such as heavier springs are not part of the Z-71 package, but a lot of trucks with the package have them. These options are not related to the Z-71 option:
G80 locking rear diff
heavy torsion bars
14 bolt rear end
towing package
heavy rear springs
push-button 4x4