Coincidently, we are studying this in Physics. I'm pretty sure the load difference between the top and bottom bolts is a Vector problem. Might also require calculating center of mass.
I think the spacers help resist bending forces and the bolts just provide clamping force. The weight applied plus the bolt torque has to be considered when calculating total strain.
While the load may not be super high, I wouldn't chance a $7,000 engine with grade 2 bolts, especially with all the poor quality, Chinese bolts at hardware stores.
It would be interesting to see what the actual loads would be. I'll have to find the equations and submit a report.
A rough calculation on the stress put on the bolts would be a ratio of the distance of the engine's c/g from the mounting flange to the distance between the bolts. Let's say the c/g of the motor is 16" from the flange and the bolts are 8" apart. If it's 2/1, the stress on the upper bolts is about double the weight of the engine.
The same rough calculation would apply to the estimate of the stress on the mounting head, the ratio of the diameter of the tube to the distance from the end of the tube to the c/g of the motor. If the tube is 2.5" in diameter and the c/g is 20" from the weld, it's about an 8:1 ratio.
It's interesting that so much time is spent gnashing teeth over the rating of the engine mounting bolts, yet so little time is spent calculating the forces on the cheap Chinese engine hoist and the low-priced chain that comes with it.
I'm with Al on this. All my life I've used whatever bolt fit and all those iron headed BBC completely assembled pushed around the shops have never failed. Even the aftermarket block and stoker cranks with my new motor was attached with whatever bolts I found and that motor added 100 lbs to the front of my car.....LOL. Now I'll have to find better bolts and a 5 minute task will take all week.....LOL. I'm filing this under the More info than necessary section. RM.