The first video looks like lean misfire to me since it comes and goes. Doesn't look like an engine balance problem. BTW, the way to tell between a 273/318 forged crank and a 340 is that the 340 has a hole drilled lengthwise into the front and rear crank throws. It appears that the crank has one in the front, which should mean that it has one in the rear as well.
As for the second video, is the rear of the car supported in front of the leaf springs, letting the suspension droop? If so, that can cause the noise you are hearing. the driveline angle will be off with the suspension at full droop. Put it on the ground a take a quick drive.
