I remember seeing prototype photos of N25 bumper on a 69 GS but never went to production. Check this 69 Riv out though. http://m.ebay.com/itm/151641739380?nav=SEARCH
I thought about doing that but the gas tank is right there on the R side. Dunno how he got that in there.
if you look at a 69 Riviera resonator, The pipe coming out make a sharp bend and exits. He probably just cut off the tail pipe and ran a straight piece right through the bumper. There is space between the fuel tank and the quarter panel to allow room for what is essentially a secondary muffler