Confirm this with others (or Edelbrock), but late model heads use a different bolt angle on the intake than old heads. They list their heads and intakes as bolt-on pieces and they assume that you would be putting their aluminum heads on with the original iron intake. We know better, but they have to cover their butts.
The old intakes/heads have bolt holes that go straight into the heads (about 45 degrees from straight down) The newer style heads have bolt holes that angle down to improve gasket sealing. It is fine to switch styles, but if you buy late model heads, just buy the intake and valve cover that is designed for them. Otherwise, they all are "interchangable" meaning that they will physically work if you use the right gaskets and components. One call to Edelbrock, FelPro, Summit, Jegs, or PAW will get you on the right track.
As far as your head choice, you have many options. Here are a couple sites for you. they list heads and their factory flow ratings. Here's the best way to describe it:
1) find your desired hp and the rpm you want for the hp peak.
2) have your cam's peak valve lift (lobe lift times rocker ratio)
3) visit the bottom of the first site below to determine the intake flow you need to achieve your goals.
4) visit all of the sites to find the heads that flow what you want at the lift your cam gives.
An example: I was building a 454. I wanted to make 400-450 hp at about 6000 rpms which required about 250 cfms. By visiting those sites I found that stock 049 big block heads flowed 250 cfms as cast. Perfect. Their chambers were of similar size to my current heads so compression was right on the money.
For 300-400 hp, I wouldn't go with $1000 aluminum heads. I would go with the $200 factory heads of your choice. Vortec heads are the absolute perfect street heads. One of the things you want to look for is the most flow you can get with the smallest port volume. Flow makes HP, small ports (and therefore high velocity) make torque. The "best" heads would be ones that provide your peak flow with the smallest port volume. Vortec heads are supreme in this area. They are the late model versions and require the angled bolting, but here's my recipe for you.
Vortec heads, out of the box, bolted on. Edelbrock 2116 Performer vortec manifold. I might suggest a 1406 carb just for the electric choke. Its an extra $20 but you'll thank me for it later. Edelbrock makes a wonderfully easy to tune carb that performs very well. An even slightly better choice is a Q-jet. Trust me on one thing. Pick up the phone, call Jet Performance, order a Q-jet and tell them your cam, intake, and head choice. They will send you the worlds most perfect carb that you can literally bolt on and know that you're getting the best mileage and performance you can get. I've tested this several times. I used a BG street demon on a caddy 500 and got a respectable 390 hp and 13 mpgs. I ordered a Q-jet from Jet and I now get 412 hp and 18 mpgs, and the carb was the ONLY change. The Q-jet is just a better design in all areas. Don't try to set it up yourself. I'm a seasoned tuner and I don't have luck with Q-jets
You'll hear terms like Quadra-junk and Quadra Bog, but they are derived from the fact that they are very complicted to tune. There is a reason that Edelbrock makes their Performer intakes with a Q-jet style plenum. I would use a Comp Cams Xtreme Energy cam in an XE-268H grind. The Xtreme Energy line uses the most recent technology to maximize low end cylinder pressures and high end breathing. The bottom line is that they can use less duration to make more top end hp while making less sacrifice to the low end torque. Its a flatter torque curve and it pays off big time.
That combo should get you easily to the 390-410 range. Your horsepower should peak at about 6000 rpms, and should put you squarely dancing around 400 hp. Your torque will have to peak around 4000 to get that kind of power, so a converter stall increase is definitely in order. The torque curve is relatively flat up to 4000 (it will also dance around 350-400 ft-lbs) so I would suggest a 2800 stall converter. You'll lose a good deal of MPGs, but if you want 400 hp my guess is you don't really care about mpgs
Sticking with the 600 cfm carb will help. Your peak requirement would only be about 620 cfms anyway and its always better to go to the next smaller size than your requirement. Vortec heads!!! Just do it.
http://users.erols.com/srweiss/tablehdc.htm
http://www.chevyhiperformance.com/techarticles/41598/