I've thought about putting one of those on our Serengeti Puma. There's plenty of room, so I doubt that would be a problem. Is the GX340 the 11hp and the GX390 the 13hp? Seems to me you could mount a 13hp engine with a torque convertor, header, air cleaner, and a smaller rear gear and end up with torque monster with great acceleration and good top end. No worrying about governors or internal mods. Is there anything special about lining up the TC or jackshaft with the rear sprocket? The larger engines are about 3 1/2 inches wider than the 6.5, so I'm guessing you'd have to bias the engine to the right side a bit. I'm a little afraid that the bike would be a bit unbalanced, and want to lean to the right. Comments?