408 stroker rpm questions

There is no better to expect with stock heads and a small carb. What works on a 318 is MUCH to small on a 408.
Almost no matter what cam you put in there... it will just make the midrange peak higher (someone said light switch power
recently).
In order to make the engine rev higher you have to do something to make it breathe... and that is cylinder heads. Ported iron
or some aluminum heads would be the first step. Even stock aluminum heads like the ubiquitous Edelbrock heads will be done
by 5500RPM most likely ... so you are getting the picture.