BVR421: "HIS..solid factory type mounts to factory frame"
More than just my opinion:
That would be the Worst-Case method to induce cracking-force on the #2 & #3 Mains.
Please don't blame the Factory Block alone, because the Block and Motor mounts beome a system of torsional force loading into the Frame cradle:
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Initial Driveline Free-Body diagram would show the Trans output shaft delivering a full MULTIPLIED output torque, while the Center-Support//Case takes 3/4th's of that torque force to the CROSSMEMBER. (((I make sure my Crossmember is Stiff, centered, rubber-isolated, the Trans Tail features the stiffening ribs, the mount and Shaft Loop are mounted well and the bolts have all the threads in use. The Tail cup is a good place for Half-Inch Studs. And good 6-bolt dry-attach to the case, and the Center-Support to include the "case-saver" to reduce case wear from peening.)))
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Engine System: Local Free-body force diagram shows the Crank flange having the full ENGINE output torque, and the Bell Housing has the full counter torque. IF we stop there we desire to install a rear motor plate to the frame. But we must read on:
Since the Crank has length, and includes torsional variation, we desire to include the Block countertorque to the Factory Motor mounts with a Rear Plate into rubber. I suggest the Factory rubber mounts and Rear Plate with same (Firebird or Big Car) rubber mounts attach rear plate to frame. Solid mounting all around the engine is best if the frame is torsionally stiff. Further, only if the frame is Roll bar-d or caged then the Rear Trans mount and crossmember can be solid attached.
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We should develop a Rear engine plate to frame. A pair of 1/4" steel or aluminum E-straps having attachments at the Bellhousing 3 places, and a single strap to attach to a motormount located at each frame turn-up. Ask me how I know it will clear the headers.
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After reading this (in agreement or not with above statements) a post-mortem observation on the split engine block reveals the torque transfer path FROM bell-housing TO the solid Motor Mount pair causes the COmpressed PS mount and PUlled DS mount to peel the block apart at the Main uppers as the Bell housing remains super-stiff. Crack progession occurs at #2 and goes back to #3 #4, the Bell, then #1 may go. Assume Steel Main Caps.
A rubber Driver mount, solid PS mount saves the motor for higher power levels.
Think of the Bell as a Steering Wheel and make a right turn with it. The PS mount compresses, the Drive mount pulls, the Cam tunnel in the middle wants to pull-apart or separate when the front Driver HEAD-CHAIN or Solid Driver mount are used. We think a stiff front Block web would save but the Motor mount is not pulling from #1. Further a Front Motor plate may be shown to increase the bow-force of separation at #2 & #3, much like a front DS Head-Chain does. We could learn something from the stiff Chevy valley cover or keep our Iron Intake COOL & bolted-down firmly. More effective to include a rear engine plate.
I won't go to a front plate for reasons related to Free-Body Diagram dictates, reliability, and leaks. A stiff Front plate with rubber mounts or perhaps steel mounts might also induce cracking without a rear plate. I still wonder if the Front Engine plate is worth anything.
HIS