Using vacuum gauge to tune engine?

I'll tell what OP,
that rocker clatter should not be there; you need to get that sorted, because, your cam is only a 284/484 when the lifters are working right.
Also
I had a 292/292/108 cam in my 360 that idled down to 600 and less in first gear with a 4-speed and 3.55s, pulling itself along, with the timing set to 5*.
The point is; give the engine what it wants. Not what you think it needs.

Your idle quality, at 1000rpm, is telling you the tune is sick.
It should be idling;
ba-bump-bump-bump,bump-bump-bump- ba-bump,
or even better is something like
ba-bump-up, ba-bump-up, bump,bump- ba-bump-up.....
ticking over at 700/750;
I don't have that cam anymore, and the replacement, I like it ticking over at 650
This is not rocket science.You just need to get the transfer slots and mixture screws synchronized, no big deal.
But that cam (in a SBM) is gonna want some bypass air, more than what the PCV can provide,and it will NEVER be happy until you put a big V-can on it.Here's the deal; your mechanical advance system can only provide the correct advance at two times; the first is at WOT, after it is all in, usually after 3600rpm; and the second is never at no other time. At ALL OTHER TIMES THE TIMING WILL BE WRONG.
Now, you can try and get the WOT timing close, at WOT, by dinking the timing curve and initial timing around, but you cannot ever hope to make it perfect below about IDK 2800 rpm. And that mechanical curve is DEAD WRONG AT ANY OTHER THROTTLE SETTING BESIDES WOT. That is where your V-can ATTEMPTS to provide the missing Part Throttle timing. Namely from idle to about 3600.

If you don't believe me, do this.
Start your car and warm it up. Loosen the D, and start cranking some timing into it keep on cranking until the rpm stops rising. Now read the timing. This is the correct amount of timing at that rpm and load setting.
What did you get?
I tell you what you didn't get; not5, not10, not15 and not even 20 degrees.IDK about BBs but my 367 likes in the 30s/deep 30s . By 2400 it is liking 40s /deep 40s, and by cruising speed it is liking 60s!
Meanwhile;
My 360 with the 292 cam, and the T-slot synced;
liked one bypass hole drilled in each primary throttle blade. I put it on the front part, between the transfer slot and the mixture screw, about 1/4 inch back from the edge, and chamferred both sides. This was my first time so, I started with a 1/16 hole and ended with 1/8 (in each). I discovered that 1/8th was too big, and I couldn't idle the engine slow enough and still maintain the T-slot sync. So I had to solder up the hole and move over. I ended up at, IIRC, at 6 or 7, 64ths., which being .094/.109 inch.. I can't tell you how much your 383 might need, or even if it will need any at all; you won't know until you get the lifters quiet, and Transfers synced up, and the timing backed up to set the idle speed where you want it.
I highly recommend to leave the vacuum gauge in the tool box; it will just drive you crazy..