Diameter of 68 Valiant 170 single exhaust system?

2" will work, might have to cut slots in the end of the muffler for it to clamp down on the exhaust pipe - all depends on how tight the fit is between muffler and exhaust pipe to start with. Just remember, there are no dumb questions. We all had to learn this stuff by asking questions and trial and error. Just don't give up.