I think its funny a guy that estimates his in flight rpm and tells another guy that has telemetry showing real in flight numbers that is over revving the engine.
To Brian the OP start with the 18x6 get some flights on the plane and then try some other props and see what you like. Sometimes a big low pitch prop doesn't climb the fastest. What I mean by that is if you have a normal big plane like say cub a prop in the 6-7 inch pitch range may climb better than a prop with a 4-5 inch pitch. The plane will fly faster on the large pitch and climb better as it climbs on the wing. Look at the weight lift competition planes built by colleges and most are in the 5-7 inch pitch range on direct drive engines. One other thing to look at here is your flying style is different from everyone in here so find the best prop for your plane and your style and enjoy.

Talking about your mustang and how to shift it from 3rd to 4th, how does that help the OP?