RE: Electrifying a great planes extra300 .46 size
DriveCalc is a great tool for seeing what happens when you change one parameter, like pitch. since you are new to EP learning how to use it will be a great help.
I'm not sure why you think a 6lb airplane is too heavy for EP, unless the wing loading is very high. Even so, if it can be flown IC it can be flown EP. I have been flying electric exclusively since '92 and have never let the intended power source stop me from flying a plane with EP. Ex: top flight P-47, Nosen 1/4 scale Citabria, Platt Waco, Sig Astro Hog, GP Kaos 40.
As for props, the VERY rough rule of thumb is "inch for inch." Increase the dia. 1", decrease the pitch 1".
Hope this helps. Keep asking questions. The only dumb one is the one you don't ask.
Walt