RE: magnum with sj 50
Be careful with the full size pipe.
Most stock pipes do not produce enough backpressure for the jett engines. Engine may go lean in the air. Dub modifies the pipes he sells. You can fly it with what you have there, just be careful. Also from the data you have there, it looks like the pipe may be set just a bit short for the bigger props.
From what you have shown, I'd stick with the SS muffler and that 8x9.25. The engine will unload over 20K in flight.