"I heard" is usually followed by complete BS, as it is in this case. You now have reason to doubt anything that comes out of that "expert's" mouth.

Fact is, ANY bolt will stretch the same amount with the same load, until it starts to deform, usually by necking down. The reason to use a higher grade bolt is so you can tighten it tighter, providing more clamping force.

If, as I suspect, the head bolts that originally were in the 413 were Grade 8, then ANY grade 8 bolt will substitute perfectly well. And I really don't see why they need replacing in the first place. I've been reusing head bolts and every other kind of bolt for 40+ years, on stock type rebuilds. Actually I have only built one engine on which I replaced head bolts.

Your low compression industrial 413 isn't a high performance engine and I'd reuse head bolts without thinking twice.

As I said above, a Grade 8 bolt will work on any application in your engine EXCEPT for rod bolts, which are a specialized shape anyway.

R.