bet it broke right at the main joint between cyl 1-2 and 3-4. Like this
I have seen/heard of 3 different cranks break in the same exact spot, and that is just around here.
My crank broke in that spot about halfway through 2nd gear on a 1/4 mile pass. I was making around 780whp through a th400 at the time, and im willing to bet it was actually putting alot more down at the track. All the bearings were fine, and no signs of detonation or any abnormality. The crank just broke from what appears to be shear force. I cant complain though that crank had alot of miles and hundreds of passes in the 10s and 9s on it.