I got a 383 stroker in my 85 z28.

The car only seems to start with the choke fully open and idles really rough. It also lacks power upon acceleration:
https://imgur.com/a/lUdeJN2

Car was backfiring through carburetor when I press the gas pedal

It was also puffing black smoke out of exhaust during high rpms (4k+)

New ngk 3035 header plugs, new ngk hei 8mm plug wires. However this poor idling was happening with old plugs/wires too.

car also diesels for like 5 seconds after turning it off. I reckon im running way too rich but dont know where to look. I tried advancing timing but no help there it still idles poorly.

This is what my carb looks like at idle: https://imgur.com/a/ZVw3rxM

It's a edelbrock 1406 carb
msd 6a 6200 ignition
Msd blaster ss coil
Msd hei distributor

Anyone know what could cause this?

 

Your assessment seems correct. It is probably rich. It's a carburetor, so the basic items are float level, fuel leaks, and adjustment.

Did it EVER run properly with that setup?

 

It did run much better when i got the car, but it needed headers and a lot more work.

 

Some of what you describe sounds timing related.
Have you had a timing light on it? Is TDC on the timing pointer verified?

 

-check timing with a light

-tune the carburetor(because it’s currently running pig rich)