You have to be careful reducing bolt torque. Bolt torque is a method to determine and set bolt stretch in a blind hole. This stretch keeps the bolt from loosening and coming out. It works like lock washers do.
You are correct 19DART66 in that there are 3 different expansion rates to consider. The Al is the one with the most different and highest expansion rate, but the bolt is "buried" in the Al. These bolts break either at the head to shank or just flush with the head.
The old iron heads on very rare occasion broke a bolt. I am not familiar with the manifold bolts used on the iron head Mopar engines, but Ford and Chev used 3/8". The worst problem on old engines seemed to be manifolds cracking, especially the inline six engines. That long manifold wanted to expand and contract a lot.
When Detroit Diesel came out with the Series 60, they had a 3 piece manifold. The outer end pieces slipped into the center section, 2 cylinders per piece. After a number of miles the expansion and contraction cased the joint areas to gaul up and leak. We tried antiseize compound on the joint parts but that just delayed failure by a week or two. Management did not know what to do to solve it. I proposed getting a ring groove cut into the outer end pieces and hooked piston ring type seals installed. This was done for two engines on new manifolds. Kept them from leaking but the ends expanding and contracting caused the ring seal to wear into the center manifold part. You could not get them apart later, they were locked together.
On a highway truck the exhaust is fairly cool on level ground and then gets cherry red going up hills. Going down the other side and back on level ground the exhaust cools substantially, so they expand and contract multiple times a day.
A different iron chemistry to limit expansion of the manifolds may have fixed the problem.