X is the engine code for a 390 c.i. 2 barrel carb. V8 rated at 295 h.p. in 1968. The S engine code was for a 390 4 barrel carb. rated at 325 h.p. Welcome to the forum. You should know that the Classic section is divided into two parts: Talk and Tech. You are in the Talk section and this question should really go in tech. Check it out; there is alot of good info there. Talk is mainly for b.s.