Why is my engine knocking?

My car is a 1977 Chevrolet Caprice Classic, 305 5.0L V8 engine. Its has miles on it. When I bought it, it wasn't knocking. I changed the stock 2 barrel carburetor for an after market 4 barrel carburetor. There is more air going into the engine now. I noticed a slight knock. I then did an oil/filter change. The oil was really really dirty. I noticed that the oil was still dirty, but I still drove the car as usual. Now the knock is loud and it's constant at idle, during acceleration, and under normal driving conditions. I changed the oil/filter again, but it is still knocking. I removed the valve covers and turned the car on, I noticed almost no spitting oil coming out of the rods, only slight drips, but no messy oil jumping all over the place like I would have expected. I looked under the valve covers and found what seems to be sludge-foam or something. I haven't put any engine cleaner or additives in the oil just to let you know.

I hope this helps you answer my question, Thanks!