More HP from 5.9 Magnum

magnummopar, I trust that you know what you are talking about. You pulled me out of a bind when no one else could diagnose the problem. This engine runs good but I would like a little more. I know gears would help but hate to give up the mileage. This is a cruzer and not a race car. I would be happy in the 300-325 HP range. I'm not sure, but I don't think a stock 340 was any more than that.
Tom