Some basics about FE cranks----the areas of question are the front---the area of the front main--the weakest part (and portion that no horsepower leaves the engine from) is from the 1,2 rod journal FORWARD---the only torque that goes thru here is what is necessary to drove the cam, dist, fuel &
oil pump and the belt driven accessories. The dampner is heavy, and way forward , probably forward farther than any other crank out there. An external weighted dampner will put tons of occilating stress on the crank snout and cause a fracture the other side of the #1 main between the main and the first rod journal. With this crank, 4.250 stroke, there is a .250 longer arm plus half of the rod journal size differance that is not overlapped between the main/rod journals. makes a ideal situation for failure even without introducting stresses from driveline harmonics.
However, my attention is drawn to that broken timing chain---steel roller (not plastic teeth) chains are especially prone to harmonics and it appears that this one broke straight across at the tangent tooth, which in this case would be the slack side of the chain??????