I had the same exact problem on my 331.
I used the dipstick in the stock location on the block, block had girdle. Girdle deflected dipstick into the counterweights of the crank. Crank sheered off dipstick tip. Dispstick tip went into oil pump, oil pump seized. Seized pump twisted pump drive into drill bit.
I pulled the motor, luckily I didnt damage the rotating assembly. I shut the motor off as soon as I lost OP. Everything worked great for months until I melted a pistion...but that was a diff strory a long while ago.