I have a 1977 120Y (B210), that is my farm hack, as an auto it was gutless!, but I found a hotdog/cherrybomb muffler in the shed, where the engine pipe flattens out to go under the car I cut off about half a metre back from there and put the hotdog/cherrybomb in the "cavity" under the passengers seat as an auto top speed went from 75 Km/h to 100 Km/h but now that it is a manual it goes VERY well now! also because my 120Y isn't road worthy I did some things that you might not get away with on a road car like removing hoodlining, carpet, all the "interior plastic crap", heater(it may go back in though!)

I wouldn't copy my exhaust sytem exactly just get rid of the stock muffler.