Ok, here's what I got:
327, domed pistons, 2.02 heads w/64 cc combustion chambers, LT1 cam (specs of the 70's LT1 350/350 hp cam) Edelbrock Performer EPS, Holley 750 DP, HEI vacuum advanced dizzy.
Ok, here's the problem:
I have the timing set to 6 deg. advanced and the carb seems to be tuned correctly with a vacuum gauge. 1. I can't get the car to idle under 1000rpm in park (turning the idle screw down does nothing, and there's no tension on the throttle cable) but in drive it drops down to about 500rpm and will stall unless I hit the throttle. 2. When revving the motor up slowly, it seems to work fine but when I crack the throttle, I had a nice flame ball come up from the carb.
Here's my thoughts:
Flame from the carb suggests too advanced timing, but 6 degrees advanced initial timing shouldn't be THAT extreme, should it?
Inability to get below 1000rpm, would that also suggest too advanced timing? I'm guessing the centrifugal advance is kicking in and not allowing the timing to come back to base + vacuum advance.
Everything is new on this engine except the carb and I did have it running before with an Edelbrock 600 cfm carb. Edelbrock 600 seemed to be starving the engine so I bought the Holley 750 DP. The Holley has had work done to it, secondary throttle blades each have a hole drilled in them and all things choke related have been machined off. Jet sizes are 72's and 76's.
Any thoughts? If it is the timing, what should I set the base timing to?
Thanks!

 

How much manifold vacuum do you have? Trying to make sure the power valve in the holley is pulling closed at idle conditions (idle circuit). I may be way off, but it sounds lean. I run my SBC's at 10 sometimes more degrees, but with the compression it may not like that. 6 should be perfectly fine if you are running hot gas.

 

vacuum at idle is at 15
Not entirely sure what you mean by hot gas but I run premium. 92 or 93 octane depending on where I fill up.

 

Thats what i meant. Your vacuum is more than enough to pull the power valve closed. When you got that "fireball" you could have popped the power valve unless it has a ant-backfire device in the base of the carb. Newer carb or older carb? The new ones come with that already in them since holley knows it is an issue. Got enough accellerator pump squirt?

 

Stall converter speed? With that cam you probably would need something a little softer than stock.

 

Not sure how old the carb is. It's probably under 10 years old if that helps. There's plenty of accelerator pump squirt, it sprays pretty good. What does this device in the base look like?
It's a stock torque converter, I don't plan on keeping it an auto so I didn't research much into stall converters.

 

I'd bump timing up, and try lowering idle set screw. My carbed 350 is running right now at 14*BTDC. Most folks start at about 10* BTDC and adjust from there. 6*BTDC might be a bit low.

 

that's the opposite of what I was thinking of doing. I'm no pro but if it backfires through the carb, doesn't that usually mean the timing is too advanced? i.e. intake valve isn't fully closed before detonation?
I also don't know a whole lot about Holley carbs, that's why I was leaning toward it being a timing issue. But, needless to say I've been stumped. I didn't want to just start adjusting the timing if it was a carb issue.

 

You cant see it with the carb on and its in a little hole in the base. It is a simple spring, ball and seat. When the carb backfires it is suppose to close the hole to keep the power valve from getting positive pressure and blowing a hole in its diaphram and usually does a good job. Try bumping your timing like camaronebie said. It cant hurt to try, just don't let it ping, it'll knock the rings out of it. I had mine set TOO high and broke 11 rings. Yea i know there are only 8 pistons, but I got 2 of them on some. Good luck.