BrWil, what's your end purpose for this engine? Are you going drag racing? Daily driver? Weekend warrior?
The honest answer to how you intend to use it most of the time would *really* determine your parts and build. Something that is absolutely a terror on the strip might be hell to drive in daily traffic, have no low end or midrange, and need avgas to run without exploding, for example.
Also *really* important to consider is the block. If you're building a 302 style block, your power limit really needs to be about 500 horses. Around that power level is where the blocks tend to split right down the middle. You would have trouble hitting those numbers without supercharging or nitrous using GT40P heads, but it's still worth considering if that's really where you want to be (for racing).
Here are some basic recipes to help you out.
For a great everyday car with all-around power, the 347 is king of the tiny V8's. Its torque and low-rpm grunt can annihilate tires and push you around even with mild rear gears (like 2.80's) and a 3 speed automatic. Pairing it with some cast iron K-code style exhaust manifolds and an Edelbrock Performer (or possibly a Performer RPM) with your aforementioned GT40P heads, and a good Holley 600 CFM carb (or 1.12" venturi 4100 if you could find one!) with vacuum secondaries would give you a beautiful street brawler that could also cruise highways without fuss. You'd have phenomenal throttle response at ANY speed, with power falling off around 5200 or so. You might see 25+ mpg with this setup, depending on your cam choice. a Comp Cams Magnum 270 roller cam, or thereabouts (a mild performance cam) would be perfect. I think you could realistically expect about 300 RWHP and around 400 lb-ft of torque with this setup, if you carefully port match and take time to do it right. It should live a nice long, fun-filled life. With just basic bolted-on parts, probably around 280.
For something a little more vicious, but still streetable in good weather, you could step up to the Performer RPM with some good Tri-Y headers from Dougs, along with some better aluminum aftermarket heads. TW170s or AFR 185s would be appropriate. You could still get by with a 270 roller cam, but if you wanted a rumptier idle, you could go with a bit more duration. At this point, you would need to decide if part-throttle performance and streetability is important to you when it comes to carb selection. A 600 cfm carb will be starting to hold you back on the top end, fractionally, but you would still get awesome response from the vac secondaries. Going to a double-pumper setup means that it will know just what to do at full throttle, but it'll never be as good as vac secondaries for cruising or stop-and-go traffic. Stepping up to a 650, or possibly even a 700 here might make sense. You might be hitting 400+ rear wheel horsepower here, depending on your choices, and how it all works together as a package. RPMs will be determined a lot more by cam choice, as will horsepower, now that heads aren't your power limiters.
And for full-tilt racing, you would really need to go to an aftermarket block. Why spend the money putting together a wicked engine that can blow past 500 flywheel horses, when the block is a ticking timebomb of wallet destruction? Once you get there, things get a lot more exotic. You could probably stay with AFR 185s. You could also consider Canfields, Kaase P38s, or many of the other race-only heads out there but you'd need a mean cam, single-plane manifold, your 750 Double-pumper or better yet, a 1000 CFM Holley sniper or the like. A single plane manifold makes more sense, as do equal length full tube headers, because you are trying to focus everything on a strong top-end charge instead of giving your engine good low to midrange power and throttle response. You will need solid roller lifters, since you will probably be spinning past the limits of hydraulics (they're pretty well done by 6500 or so). Your gas mileage is going to be single-digit, and you'll need some steep rear gears (3.73's or higher) because your low end is gonna suck. If you're running an automatic, you will absolutely want a 2500+ rpm stall converter. Stock's not gonna cut it. You'll also need to make sure you invest in some chassis reinforcement, and some better tires. At this stage in the game, you really need to involve a professional to help you plan it out, because a mistake in the design will be the difference between victory, or broken parts lying all over the track.
Just to give you some hard numbers on the carb stuff: a 347 spinning at 6k RPMs uses almost exactly 600 CFMs, even if you're getting 100% volumetric efficiency - which *could* happen if you have a really tight setup. Stock is more like 85% or so. Going with a 750 Double-Pumper is going to slaughter the fun factor with this engine, and it'll come off idle like a fat kid walking home from Thanksgiving dinner.
You said that the GT40P heads are ported? Hopefully they didn't mess with the runner shape too much, because those heads are *really* fussy about having stuff removed in the exhaust area especially - and most of them that get messed with actually flow a lot worse than stock, despite having larger volume in the runners. Matching to your intake makes good sense.
Lastly, I'd like to mention that all of these parts will work on a 302 just as well as a 347. They'll make similar horsepower - but your 347 with the same parts as a 302 will produce a lot more torque, and finish at a lower RPM - probably about 400 RPMs lower, typically. Your real horsepower limiting factor will be the heads. The rest of it is all a supporting cast, to ensure that you get air into and out of the motor. Most 'good' GT40P setups get about 300 horses as a 302 with a Performer Air-Gap and decent headers. Hard to believe it'll make about the same horsepower with 45 more cubic inches, but it's true! You'll just have a bit more tire smoke and be done a little sooner. I suggested the K-type manifolds because the GT40p heads require hard-to-find headers that are a bit pricey, and don't really work much better. Most manifolds will work, but fitment and performance are great on the old Hi-Po style setup.
Hope this helps, and welcome to AFM!
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