Assuming the PCV is working correctly, and, the booster is working correctly;
Do this:
1) Advance the timing until the rpm plateaus, and more timing slows it down. I don't care what the number is.
2) Shut the engine off, take the carb off, open the choke and make sure it stays open, flip it upside down.
3) using the speed screw, close the primaries up tight, then, examine the secondaries to make sure they are also closed up tight. That cam on your low-pressure engine, does not need 4-corner idle, so shut it down. With the speed screw, open the primaries until the Transfer slot is about dead square, to a little bit taller than wide, just enough so that you can see it ain't quite square.
4) Flip the carb back right-side up. and
Open the PRIMARY mixture screws to 2.5 turns out
5) restore the carb to stock.
6) put the carb back on, fill the bowl, and
7) from now on DO NOT RESET THE SPEED SCREW.
8) start it up and retard the timing until it idles at 750/800, then let it warm up. I still don't care what the number is.
9) AFTER IT HAS WARMED UP, Idle it down some more with timing, about 50 rpm atta time until it goes lean for lack of transfer fuel, or you hit 650.
If the idlespeed gets to 650, that's pretty good; bump it back to 700/750 with timing, and check your oil-pressure. If over 10 psi, go look for a tip in sag.
If you get a tip-in sag at 2.5 turns out, add 50 rpm using the speedscrew.
If it goes rich, take a little out of the mixture screws.
If the rpm is too high or too low, change with timing, DO NOT USE THE SPEEDSCREW.
10) after you get it ticking over real nice, NOW, check the timing. It should end up in a window of about 12 to 18 degrees. Somewhere in the middle with a stock convertor, going to the high side is permissible if you have a hi-stall, and
a manual trans car will run better at low mph, if set to the low-side.
11) go to the back of the car, and smell the exhaust. If it stinks real bad and burns your eyes, the engine wants some bypass air.
BTWs
1) that cam, at idle, will lie to you all day long, about AFR. Set your AFR at ~2400 rpm no load, then let it idle at whatever.
Specs for that 6900, are 278/288/112/59 overlap, 204/214 @050
2) That 59* overlap is gonna mess up your intake plenum until at least 2200, so 2400 should be safe.
3) At just 140psi cylinder pressure, You're gonna need; at least hi 3-series gears or a hi-stall, or some combination; or a clutch.
> on the hiway, that cam will suck gas big time, unless you get the rpm up, and run plenty of timing. But around town it will be really bad. That's just the way those long period cams are, running low rpms at low cylinder pressure.