Loadstar 1600 Truck Question

[Tom and Amanda Lang <rustnrot@domain.elided>]

A friend of mine has a 1972 Loadstar 1600 dump truck with 345 2bbl, 4 speed
manual tranny, 2 speed rear end.  Believe it or not, his first I-H.  He now
owns a Scout 800A, 1970 with 36k miles ;)  Anyway, for the Question.
Howard, prolly need your help here - what kind of 4-speed tranny is in it?
It appears 4th gear synchro is out.  Went out suddenly.  Grinds going into
fourth.  Is it possible the synchro could be *stuck*?  Would taking the
shifter off the top and "freeing stuff up" in there help any?  Or is it
likely to be *toast* and might as well jerk it outta there and carry to the
rebuild shop?  Would one of the Clark or I-H 5 speeds with overdrive (if he
could find one) work?  

Thanks Ya'll!

Tom (hoping it's a common type tranny that we can get parts for if we elect
to rebuild it) Lang