You need a timing light.
Use the timing light and set the initial timing at 10-12 degrees initial. Make sure you do this with the vacuum advance hose disconnected and plugged. Reconnect the vacuum line and set the idle speed to 900rpm.
If your initial timing is too low and you don't know it, you'll be beating your head against the wall trying to make it idle in gear with a decent idle speed in park/neutral.
Unless you have a huge camshaft there's no need to open the secondary butterflies any further than Holley did. Return them to the original position.
Make sure you don't have a vacuum leak anywhere.
PS, If you have a big lumpy cam and a stock or stock type converter you'll have the same problems because of low vacuum. Not much you can do to fix it but swap the cam or the converter.