1) Have you checked all fourteen main bearing cap screws to see if any are missing heads? Got all fourteen screws in place? You didn't mention that.
2) Verify the threads on the screws - I'm almost willing to bet they're 1/2"-13. The 242ci (seven-main, two-bolt caps as well) uses what appear to be standard SAE8 capscrews for the main caps, and it has higher torque and power output figures. If SAE8 is good for that, it will be fine for your 258 (if, on the very off chance it's ISO, use ISO PC 10.9 or better.)
I'm inclined to think that the "M9" is a manufacturer's mark - is it in the centre of the head? Is there a series of "hash marks" around the head - one pointing to each corner? Add two to the count of "hash marks" - that's the SAE strength grade (six marks = SAE8. Three marks = SAE5. No marks = SAE2.)
If you get confused, submit a decent picture of the screw heads (not on a cellphone camera, unless that's really all you have and it takes a decent pic with good detail...) and we'll sort out you some more.
"recon" (sic - reckon)(tm) "hihgly"(tm) "seceed"(tm)
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