the first 4 letters the vin number should be 242
2= Pontiac
42= GTO
I think the VIN for that year is located on the left front door post.
The next big thing is the engine- there where lots of options in 68.
GTO should have the following engine production code - alpha numeric(Stamped on the right hand cylinder bank.)
XM (400 -2 barrell) = 265 HP (Wierd option for GTO that year)
these last codes are for the 400 - I think the YS 400 is the most common where as the others are special order
WT = 335 HP
WW = 335 HP
YS = 350 HP
WS = 360 HP
XS = 360 HP
YZ = 360 HP
XP = 360 HP
I pulled this info. out of teh STandard Catalog of American Cars - you can find it at any book store.
Good Luck- I think $4000 if its a driver is a real good price.