I have a 37 Buick with a straight eight which had a manifold cracking problem on manifolds that are hard to find and $$$. The car has been driven 16 hours non stop at 70mph which it was never designed for in 1937 and I think the extended high heat and quick cool down was a contributing factor. I trued all mating surfaces, cleaned operating room clean, and discarded the alignment rings replacing them with an all studs-no bolts set up, applied a bead of high heat silicone, and drove trouble free for 25 years. The operating room clean is mandatory as silicone seals by adhesion, not compression. The alignment ring delete allows the manifolds to expand and contract with no binding. Picture shows manifold has been VERY hot with no harm to sealant.

[img]https://i.postimg.cc/dhYr1qKp/Pont-heater-Buick-sealant-003.jpg[/img]

Evan