I live in Colorado Springs which is about 6,200 ft and I think you are still way too rich. I usually have to step down three jet sizes to get close to what it should be. Also, Edelbrocks don't like much fuel pressure, I used to run mine between 4.5 and 5 psi. Seemed to like that better than the 5.5 or 6 like the book says. If you are not getting any change with the idle mixture screws, then it is getting too much fuel elsewhere. Does closing the choke plate kill the motor? It should. If not, then you probably have a vacuum leak somewhere. The step-up springs have no effect on idle qualities, that is for getting into the power circuit.
If you don't want to change the jets, try installing a rod a couple sizes bigger than what you have now. You will be getting close when you can actually turn the idle mixture screws and make a difference.
Colorado Springs, CO
68 Camaro... someday!
71 Chevelle SS