I have those rocker clips on there way and once I get them I will open up both valve covers and run it longer to really look at the pushrods turning and see how each one is pumping oil. I will video this as well.

By my best analysis, I am leaning towards 8 or 6 cylinder area or both. To me it does not sound like rod knock. I feel it is either piston rattle or lifters.

Something to add on our first starting of the used engine which I could not imagine having importance is when we dropped the used rebuilt engine in, it was popping through the carb with flames. Happened a good dozen times before we could figure out the issue. The accel distributor was bad with either bad coil or module. Was not getting enough spark acting retarded.

Then we put in a GM Hei that we had and it started right up.