The reason I was thinking that a regular bolt would not make that big a difference, is because all the TTY bolts (BMW head bolts) and studs (ARP head studs for BMW engine), are 'undercut'.
For those that don't know what it means, it's that the middle part of the bolt or stud has a smaller diameter to permit the stretch. Like a 3/8" bolt will be 3/8 where the threads are, then go smaller, like 5/16 for the middle section, the go back to 3/8 one inch before the head.

The OE bolts on the stiffening plate are not undercut, so I don't think they will strech as much or as easily as an undercut bolt, or may be it's the grade of steel that is lower to permit the stretch?

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