The OE 6 packs idled at ~1100 and had a solenoid to shut the blades when turned off.
That's a fairly big street cam (yeak, someone is going to come on and say 700 is big), so idle vacuum is going to be low and you will want the idle up a bit. Problem is dropping it into gear if auto.

Here are a few things to try.

Make sure the timing is rock steady at idle. There is a "hose trick" in the tech archives.

Make sure the vacuum and cent. advance isn't coming in. 1100 is usually enough to uncover the ported vacuum. You can try to open the outboards a bit. This is a balancing act.

Now that you have a solid timing at idle, with nothing creeping in, make sure there are NO vacuum leaks and the throttle blades and shafts are tight and not hanging.