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The reason the girdles help, is because when the piston is at (or near) TDC, the rod is trying to push the crank out the bottom of the block. The majority of the is held back by tension of the main sadles. But if we look at the #1 rod as an example, there is more then just tension in the main sadle. The bending moment of the crank at the #1 cap tries to bend forward, and the #2 cap goes rearward. It’s this flexing forward and rearward that can contribute to the bulkhead breaking above the main caps.

Think of it this way… if you had a pretzel stick, and you tried to pull it lengthwise, it’s fairly strong. But if you bend it, it can easily crack. That “Bending” is exactly how the block acts. Cast Iron is not very good at “bending”, so it can lead to cracking.

Here are some pictures to explain this “bending” is happening. This is a picture of my main bearings from the last time I took the motor apart (small block, but it makes the point). On the inside edge of each main bearing, ONLY in the area opposite of each cylinder (not all the way around the edge), you can see where each main bearing was wearing. This is because the cap and crank were actually bending forward on the compression stroke!

I don’t think any of us could say if removing material from the side to add cross bolts is a trade off worth doing. It would certainly help the cap from wagging front to rear, but at what cost to the block? The girdle however would give you the benefit without a doubt.

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I disagree on your thoughts on the crankshaft trying to push the main caps forwards and backwards by bending the crank on the firing stroke, if that was the case the thrust bearings wouldn't last would they Also when the motor is firing on the power stroke the rod is pushing the against the cranksahft at or around 30 to 45 degree sideways trying to push the crankshaft mains out towards the oil pan rail, not straight down or forwards or backwards in the main saddles, correct Metal transfer(fretting) from the block to the main caps is usually cause by detonation from a bad tune up, it can be RPM related also Most failures in race motors are caused by some part being overstressed or a flaw in the metal or machining in my opinion