From wikipedia:
A popular modification for the flathead was conversion to an overhead valve configuration, and many such modification kits were available, including the Ardun heads from Zora Arkus-Duntov who was to go on to fame as the "father of the Corvette". These conversions were not initially demanded by hot rodders looking for extra power, as they had not yet exhausted the capabilities of the flathead configuration, but were demanded by users of the engine in trucks and other such high load applications, where the constant flow of hot exhaust through the block to the exhaust manifolds caused the entire engine to overheat; the overhead valve heads routed the exhaust out more directly, and away from the block.