this is easy one for sure, jsut go to auto zone or napa or equivalent, they sell lengths of 3/16" and larger lines with the threaded fittings and all on them, the fittings on these lines they have usually always have the right size and threads for the older gm trucks before '88(if not later years also i dont know when GM changed ot metric on RWD and trucks) im sure
im pretty sure 4x4 front are 3/16" just like they are on front 2wd and '60's/'70's cars at the very minimum
the stores sell several lengths of various sizes of SAE sized steel brake tubing, very inexpensive, way less than 5 bucks a piece for all of them cept maybe the longest one or two possibly,
and if you rent the SAE(3/16" on up) double flaring tool kit at auto zone for free or wherever you can then you can change length and also swap the threaded fittings for the original ones if you need to, if are in good shape still anyways. ive done it that way many times, never had any problems ever and is easy to do youre done in a matter of minutes.
use big channel locks or something similar to crank the flaring tool clamp block shaft handle tight so that the tubing wont slip on you, sometimes they dont grab the tubing so well
good luck