wow a car muffler???? you do realize that the standard car muffler has a 2" i.d. inlet and a 2" i.d. outlet right? The h.t. pipe is about 1 inch in diamater...I have no idea how you could adapt a car muffler to fit without it looking horrible. And then you'd have to figure out a way to support the weight of a car muffler (which weighs about the same as the entire engine on your bike).
Plus a car muffler would be overkill.
yes, you can bend a pipe to go all the way to the back of the bike, and a h.t. does need some back pressure.
BUT, consider this....
You would have to make a flange and weld it to the pipe, to bolt the pipe onto the engine OR cut off your stock muffler and weld a new pipe to the stock pipe to extend it.
all you need to do is weld the stock muffler back onto the extended pipe.
but if you want it silent, that won't do it.
OR you could just spend about $30.00 and buy a long pipe that's already made with a flange already welded on it AND it comes with a brand new stock muffler.