The carb idled at about 1400 rpm. I turned down the idle adjustment screw quite a bit and was only able to get it down to 1200-1300. All 4 butterflies have a hole drilled in them, and are cracked open at idle. Will it be possible to get the idle down to about 1000 and should this be a good carb for this engine (400 block, Boss 351 SVO heads, solid roller, Torker, 12/1 or so compression, 4.11 gear, toploader)?
Both the front and rear throttle blades have an adjustment for idle. The rear is just small. If you look, you'll find it.
The edge of the blade covers up a slit in the baseplate. Start off with the blades covering up most of the slit. The exposed portion should be roughly equivalent to a square... not a long rectangle.
The holes in the blades are for extra airflow at idle. If too much of those slots are exposed, it overwhelms the idle circuit. The holes allow the blades to be closed further, and still flow enough air at idle.
Carb likely has a 4-corner idle. Adjust all four screws the same number of turns out. Turn them all in all the way in until they lightly seat... then with the engine fully warmed up, turn them all 1/2 turn out and see how it idles. You will then turn them in and out in 1/8 turn increments until you get the best idle. This can be determined with a vacuum gauge, idle speed, or just by ear. Adjust them all the same number of turns.
If you have a significant vacuum leak, they may be hard to adjust.
4-corner idle (carbs with four idle mixture screws) will usually end up around 3/4 turns out. 2-corner (just the front barrels having idle mixture screws) will usually end up around 1-1/2 turns. These adjust the fuel mixture for the idle circuit only. Out is more fuel, in is less. The carb jets take over when you are past 1/8 throttle.
This being said, lack of revving past 5-6k rpm is rarely the carb... but, hopefully it helps!
And... as for your particular case... I'd probably recommend you just turn the idle down a little on the rear baseplate, and call it 'good'. THIS being said, if it's got a solid roller, you don't want to go down below 1200 rpm, as solid roller lifters lube from oil being thrown off the crank... and the extra idle speed helps. A decent sized solid roller isn't going to idle very well much below 1200 rpm, anyway.