BooNDocKs68

Hey Everyone
I am new as I am looking for some insight on what to do next. I have a 68 camaro with a mid 80's 4 bolt crate motor. It is a 2 piece seal so I believe 85 or older. I'm running a Muncie 4 speed with 4:56 rear currently. The motor was built with forged lower end and a Holley top end kit. Systemax 400hp kit I believe. It has roller rockers aluminum heads a mild lunati cam and hooker comp headers. I am running the stock hei and a 650cfm carb. Compression and leak down came back decent, nothing too far off. Fuel is at 7lbs. I haven't had it on a Dyno but I think it's safe to say it's at 400hp peaking around 5500rpm. I've been driving the car for the last 2 summers and it runs great. Everything is working well and the car really moves. It's just not enough anymore. I am tired of max cruise speed being 55 or so at 3500rpm. I'm leaning towards 3:90 instead of 3:73 or 3:55 as I don't want to loose too much of the "pull". I also don't want to tear down the motor as I see it living a good life the way it is. Maybe add a super charger or turbo, twin turbo? Just NOS? I don't know what I have for options in any of these categories. Can I run over 5-7psi? Do I need to? The motor like to rev and rpms stay up so is turbo a good idea? The car is a weekend warrior and I definitely hammer on it. I am thinking I would be happy with somewhere in the vicinity of 600hp. So my questions are how do You purpose I get there and with that power would the car be better with 3:73 rather then 3:90? Is there much difference?