broken exhaust studs

on my 1964 dart slant 6 of course. it had a bolt from ext manifold to pipe. so i put a 3/8 bolt in. it kept getting loose. i figured out the bolt from the factory a 7/16. it stayed tight then. and i like to use anti-size compound on ALL bolts on ext , and the under side. i dont think grade 8 is really needed. cause if the threads strip you loose the threads in the manifold, not the bolt. if only the bolt strips you still have the threads in the manifold. this bring me the big Q i always ask. how did the factory do it? iam not saying they never goofed up. but a lot of they did was done right.