I mounted my GP460 engine upside-down on my Scooterguy frame-mount adaptor. That left a LOT of room topside, so I installed a Happy Time expansion chamber system and ran the pipes between engine and the bike's top tube. Everything fit right in there, so I had room to install a 4-liter Happy Time fuel tank.
BTW, the exhaust bolted onto the 460 engine and fit snugly onto the bike frame. Performance-wise, it REALLY awakened this high-rpm engine.
And now it's quieter than a stock muffler.