2017 ram 2500 broken manifold bolts

Youse is overthinking da sityatun. Why do you introduce Gr 8 bolts? 9mm 8.8, 10mm 8.8 or 3/8 grade 5 bolts would be fine. IMHO the breakage is not from tension expansion of the manifold. Those bolts get plenty warm through heat convection from the bolt head in contact with the manifold. My belief is that the manifolds expand lengthwise putting a bending stress on the bolts. Hardened washers with antisieze between them and the manifold to aid slippage along with the larger diameter bolts that have the strength to resist the bending forces is the cure.
I really doubt that if the threads have the correct engagement they will not pull out of the Al heads.