Looking for Old School Mechanical Advice!

My 84 K20 has a 502 BBC, it ran strong then started dying at stops. It got so much worse it doesn't even run now. I changed all tune up componets, even put a brand new distributor on, same thing. I assumed it was the fuel injection so I took it off and put on a Demon 850 carb. STILL, same thing. It starts great, fires right up with the turn of the key, if I play with gas pedal I can keep it running for a sec but white smoke blowing out tail pipe. ALL fluids are perfect, no antifreeze in the oil. New fuel pump and regulator and definately getting fuel but running terrible. I've checked everything. It is fouling plugs pretty quick. I've changed them and also cleaned them twice. Anyone have educated guess? Pls explain reasoning too if u would.

Just a background on how it started: Went from dying at stops to dying when slowing down then trouble restarting. As long as I could feather it through with one foot on the brake, I got it home. Always starts first try just hard to keep running.When I got it up to 30+mph, it would run great! then again slowing or stopping, wants to die. It Now extremely doggish. Assuming the smoke is from sitting, been working 12s so little time to mess with it. Thanks for any suggestions!

What FI system, and what’s the regulator set to if applicable? This problem existed before the replacement of the regulator? Assuming no vacuum leaks and strong, well-timed spark, my money’s on a fuel problem that involves the pump or the regulator.

Do you have a pic of the fouled plugs? That would help with the diagnosis

Have you done any compression or leak-down checks since the issue started?

Manual or electric fuel pump? Have your tried blowing out the fuel lines? Drop the tanks to see if there is any crud in them?