[Steeveedee]

Use bolts that have at least the same penetration into the block as the size of the thread. For a 3/8" bolt, that is (obviously) 3/8". That's an engineering standard. If you want to use a flanged bolt, that is fine. But I've removed and replaced literally hundred of bolts that mate a trans to an engine, and seldom have seen bolts with flanges. In the mechanical design world, we call it "spec creep", where someone sees something that might make the product better, but ignores the engineering aspect of a design...which drives up the cost with no benefit.