My 99 Buick Century now stalls right after start

I bought a 99 Buick Century for $300 - 90k original miles. (yes, a good deal
I'd say) It's a GM 3100 engine.
The car was in bad shape, overheating and leaking coolant so I replaced the
whole cooling system and the problem was gone. I decided to restore the car
into a better shape by replacing the head gasket(and all other gaskets along
the way). I've done few head gaskets replacements on other cars with
success so I figured why not.

The problem now is the car starts and stalls right away - something the car
didn't have before this restoration. It dies everytime immediately, and
sometime it ends with an ugly screeching-ish sound and the driver side
engine bay gives me a slight burning smell. Last code reads P0300 for
random misfire now.

Spark plug wires are all new, I replaced one of the ignition coil, but again,
this wasnn't happening before the repair so I know it's something stupid that
I've overlooked or done during the repair..
The PCV breather hose that attaches to the valve cover(the back one)
seems to be loose, as if it's missing a grommet or something so it doesn't
feel secure.. so I am hoping it's a simple vacuum leak? I am going to
troubleshoot further tomorrow.. but my question is - if it's just a vacuum leak,
where is this ugly screeching sound coming from, along with the burning
smell? Any clue? It sounded ugly and it scared me so I didn't try turning
much more..  I didn't touch the timing at all btw.

Thanks for any insights everyone!