Just looking at it without knowing your grain bill or OG, I would say 6 oz of hops is a lot. You could get away with a lot less, maybe even do a 90 minute boil with half the hops. You are well over 100 IBUs with that, I like to try and be around 70 IBU. Also anything over 100 I dont really know if you are getting anything out of it. I guessed your Magnum was 12% and centenial 10% and OG of 1.100. I prefer the maltier Stouts though.
Magnum is a great stout hop (magnum, warrior, columbus are great neutral bittering hops) so I think you made a good choice there. This one would hold up better to aging a bit longer, but I know mine never last long enough.
Maybe same hops but 2oz, 1oz, 1oz? It will make a good stout, just how much bitterness and hop forward will it be??? I have had a few 90+ IBU stouts, I just havent liked them as much.