Using vacuum gauge to tune engine?

As to your rocker clatter;
You did not say that you had a solid lifter cam, or at least I didn't catch it. So Assuming it's a hydro, IDK how much is acceptable on your valve train. I have never owned a BB, nor a 284/484 older Mopar cam. Hopefully, someone who has, will chime in.
All I know is the 292/292/108 older MOPAR hydro cam in my 360 was dead quiet, with a half a turn preload on the Hughes lifters I was running.

If you have a hydro cam, and you run it with lash, two things are gonna happen;
1) it will be noisy, and
2) it will idle like a bigger cam, and will want to idle faster than usual, to curb the overlap-cycle from wreaking havoc in the intake.

From the sound of that running engine, I think your vacuum gauge is too heavily damped. What I see and what I hear are not congruent to me. I think it is reading an average number, and it's not being helpful, which is why I suggested to leave it in the toolbox.

So; IMO, once the lifters are quiet, then your 284 cam is gonna act like a 284 cam, and then the plenum will clean up, and then you will be able to slow the engine down
Oh yeah, make sure the oil level in the pan is NOT too high, and that it is 100% just oil, no gas in it.