The options that one could order on the '68 GT/CS when it became available would be essentially the same as the options available on any other '68 Mustang of the same time frame. Small differences were present when there was a circumstance where items may conflict in terms of the content of the GT/CS package, i.e. badging, stripes, grill mounted lights (GT fog lights vs the Marchal/Lucas lights in the case of the GT/CS), things of that nature. I always found it puzzling that they used the term "GT/CS" when the GT equipment group was not a standard part of the California Special option. As Royce and others have pointed out, any engine/transmission available in the "regular" Mustang of the time could be had in the GT/CS.
Sounds like a neat car. I do like the Seafoam Green, it's not a color often seen on any '68, let alone a California Special.
Paul Newitt's GT/CS books are pretty decent, the original (and now hard to find/pricey), as well as his later effort. Well worth the price to buy the newer version, IMO.
Terry