I tend to agree with you that a bit much is made to do about TTY fasteners but perhaps not in applications such as this, a relatively small screw in a highly stressed application. Indeed head bolts/studs are subject to heat and stress cycles but so are the connecting rod bolts - plus bending loads.
This is exactly the type of application where you want to ensure that fasteners are installed to a prescribed torque that yields (haha) a preload that will never be exceeded during the life of the bolted joint. Otherwise there's a chance that the cyclic loading will cause that preload to lessen and the joint to become loose.

Now, looking at ARP's installation instructions, they seem to throw much of that out the window when they say if you don't have a stretch gage, just go ahead and torque to 50ft-lbs. I suppose that's more than Mother Volvo's 15ft-lbs plus 90deg. Which would account for their NASA grade superalloy...

1.5 times the nominal thread diameter is a good rule of thumb for thread engagement to ensure that the fastener fails before the tapped hole (which is always what you want; a screw is more easily replaced - except maybe in the case of a 1996 Volvo 850 connecting rod!) The formulas governing that calculation can be found in Fed-Std-H28. The driving factor is the material strength of the internal vs. external threads. A mismatch such as we have here, a high strength fastener and a relatively weak aluminum connecting rod will require a longer engagement to develop the full strength of the screw.

I find it odd that the Volvo online parts catalog lists zero engine fasteners. We must be missing something. I think a call to the parts desk would be in order. I wonder if, although they don't list it for the 850, it is the same as any of the later white block rod bolts. The connecting rod prescribed for the 850 is also listed as applicable to 42 other Volvo models, including 4, 5, and 6 cylinder versions up through 2005. The connecting rod bolt Volvo PN 9125471 cross references to many of those models. That might give the OP's local dealer parts desk something to start from. Or if not, it may be, like ARP's installation procedure, close enough.

Will, currently a Honda Odyssey man!
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XC60 / Odyssey