Welcome to LTG!
Specifications for the JF-4 probably included a nut width of 1-11/16", which was standard for most Guild acoustics (6-string) during that period. Guild necks tend to be C-shaped in profile. However, much of the shaping of the neck was done by hand (using templates, but...) so there was some variability from instrument to instrument. I have had '90s Guild acoustics with very slender neck profiles, and also with much thicker profiles. I also own 3 that have nut widths much closer to 1-3/4"... So, if width and shape are big concerns, you might have to try a few to find one that works for you. Just because one JF-4 has a neck might be to narrow or too thick, another of the same model (even the same year) might be more to your liking...
I don't recall if the JF-4 had a 3-piece neck... it might be listed in my copy of Beesley's book (I'll check when I get home). Or, perhaps Hans Moust will chime in...