The way bolts are made, the steel is relatively soft when the blank is headed into a bolt and the threads are rolled in.
Then the finished bolt is heated and quenched to bring the strength up.
Much of the strength is in the outer shell of the bolt as that is where the bolt is hardest. When the original outside is cut away for a smaller diameter and new threads are cut in, as opposed to being rolled in when the bolt was manufactured, the new threads will have less strength than an OE bolt of the same thread size and diameter. I think the torque on the rocker shaft mounting bolts is 25 ft lbs, but keep that in mind and don’t over tighten it.
The reduced strength could be avoided by turning out a sleeve to the correct diameter and brazing or silver soldering it to the under head of the correct size bolt.
