Thanks for the ideas guys. As far as length goes, do you guys simply put the collector where you want it and connect the dots netting whatever length it takes (obviously within reason).

Are 2" primaries adequate for 600hp?

Depends if you're building dyno headers or car headers. For an early A body header you'll be lucky just to get four tubes into a collector regardless of length, size and number of bends. Those cars are horrible for headers. If you were building a set of dyno headers then 2 inch would be too big for 600 hp and the length is a critical dimension. For an early A body car all the theory goes out the window and you are just left with what you can fit into the car.

Depends if you're building dyno headers or car headers. For an early A body header you'll be lucky just to get four tubes into a collector regardless of length, size and number of bends. Those cars are horrible for headers. If you were building a set of dyno headers then 2 inch would be too big for 600 hp and the length is a critical dimension. For an early A body car all the theory goes out the window and you are just left with what you can fit into the car.

Buy yourself a program called pipemax, input your engine specs, and it will calculate the best tube diameter and length, as well as collector length, it will do single diameter or step diameter headers. It also is a good engine performance program, and does tons of other engine calculations. I have built several sets of headers using the program.
http://www.maxracesoftware.com/

Absolute truth.
These are 1 7/8 " primaries , but I took a dull axe to the inner fenders to make room , and it still got really tight on the driver's side.
I did get a nice straight shot before the first bend , and they will go on and off without too much trouble.
Still had to run one tube on each side outside the frame rail and wound up with longer primaries than optimum to get them into the collector.............33 " to 34 " long was as close to equal length as I got , but since I am running mufflers and an X pipe , I don't think it cost me too much in the way of power.
But with the kit , the flanges and coating , I was into them for somewhere around 700.00
And well north of 40 hours............maybe 48 ?
This was my first , and hopefully last time building my own.
If someone could do up a set for 1200.00 , I'd throw in a case of beer and my sincere thanks.

That's kinda what I was thinking. Space is definitely the limiting factor. With the inner fenders and torsion bars out of the way, it'll be a tad easier though. Do you think I'd be better off going with 1-7/8" primaries? I'm not going for every last hp.

Combo will likely be a pump gas 470 w/ standard port window Edelbrock RPM heads cnc'd by Modern cyl head, ~260-ish * @ .050" solid flat tappet.

I assumed 2" primaries would be an appropriate size. What do you think?

Measure the area of your exhaust port and go from there. I bet 1 3/4 will be big enough for that combo.

engine masters has done a number of tests recently on header type, size, and exhaust length. It's a good watch in my book and great background info to have before building your own IMO.