Have you checked the travel of the clutch pedal before it engages? Do you have
to come up off the clutch really high up before it engages or feels like it is?
I can generally remember something (I think it even says this in the owner's
manual of the '79 T/A I have) that you should be able to use finger pressure
and push the pedal down to a certain point and that will tell you if it's out
of adjustment or it's worn.

(I'd have to check the shop manual to be sure...)

A clutch isn't too big of a deal on this thing. It's rear wheel drive,
everything is accessible and parts for it are dirt cheap.

That is an extremely simple simon truck. Not a powerhouse, but it will bury
many other vehicles out there with longevity and reliability long after the
others have given up the ghost.
It likely has a New Process or Isuzu 5 speed in it. I cannot recall which in
that year.The Isuzu pickup and the S,T series trucks share
parts/designs/similarities over the years.
Joe--ASE Certified Parts Specialist & 10th Ann.Club Tech Director
'80 Carousel Red Turbo T/A, 27k orig.
'79 "Y89" 400/4 speed 10th Ann. T/A, 57k orig
'84 Olds 88 Royale Bgm 2 dr, 307 "Rocket" (lol), 143k and still going....
'91 S10 Blazer 4.3Z