Engine knock?

I have a 360 Magnum swapped into my 1980 volare, and the motor had about 150,000 miles before I tore it down and put in new gaskets and the like. I didnt rebuild it because it was in excellent condition, still had cross hatching without any noticable wear on the cylinder walls. I started it last night and immediately it started makinging this sound-
https://youtu.be/fmBkdXCGZ-w
I know it already has a gasket leak at the header, but I didn't think it'd get this bad. I pulled the plug wires off one at a time and the noise didnt change and it seemed to go away with increasing rpm. I'm really crossing my fingers that it's nothing in the bottom end, but I'm wanting to get some others opinions on what it might be, any suggestions would be appreciated. I can get a better video today if I need too
Author: cmrris