The bolts are definitely not the weak link - a 3/8" grade bolt has a tensile strength of about 15,000 pounds at the shank, (based on 150000 psi material). The actual strength is less due to the thread root diameter (the smallest cross-section of material). Using a safety factor of at least 5, there is still over 1000 pounds of working strength at each bolt. With 4 bolts, tightened properly, they won't break.
Similarly, as the Hot Rod article demonstrates, even a cheapie stand will take several times the rated weight before failure.
As Powerband says, the real safety hazard is the high center of gravity and small footprint of the stand, resulting in a significant tip-over risk. The extra length of the inline six aggravates this condition, resulting in even less stability.
Bottom line, be extra cautious when rolling the engine around, or when tugging and pulling on it to rotate the block, torque bolts or what ever. If it starts to tip, get out of the way!
Work safe!
-Stu