Buicks were all overhead, just like the Chevy. In fact, early Chevrolet advertising (after they bought out GM in '18) used the similarity by stating that the Chevrolet used the "Buick style" head. Olsmobile 6 and 8 before '47 were flatheads, as were Pontiacs up to '54. I don't beleive that Pontiac continued the 6 after they introduced the V8 in '55.
The closest things that Chevrolet ever had to a flathead was the '12-'13 Classic 6, which used a T head engine, and a prototype light reconnisance vehicle for the military in WW2, but it was powered by an Indian motorcycle engine, not a production Chevy engine.
