The odd looking Chevrolet chamber was copied by Toyota, and both Mercedes (L4, L6) and Jaguar (V12) produced a similar chamber.
The original Chevrolet goals were intake swirl, knock suppression by quench, and high turbulence. A "pure" hemi sounds like a good idea (first used before WW1) but it has compression ratio problems (tall heavy dome needed for high CR), bad quench, very heavy and complex valve gear, and high cylinder head weight.

W/r/t: "GMC 228 & 248 engines in the mid thirties had very radical cams -- like over 300 degrees duration"
Never saw that, source is?

The OEM GMC chambers follow 2 basic shapes, either open and roughly hemispherical (more "bowl" shaped) with no quench area (circular gasket outline), or more elliptical with quench at two opposed closed surfaces flush with the gasket.
However, "hemispherical" usually means that the valves are separately angled to the bore axis (the "stem angle", which may be equal intake vs. exhaust, or slightly different) and to each other (the "included angle", equal to the sum of both stem angles). In older engines the stems are frequently 45° to the bore, making the included angle 90°. This includes H-D OHV engines 1926-83, Chrysler-Dodge-DeSoto hemi 1951-58, Triumph and BSA motorcycle, Jaguar XK-E, etc.
By comparison, all 4 of the OEM GMC head types (hemi or closed, big or small port) have vertical and parallel valves.
The Wayne GMC head sat flush on the block (not wedge or hemi), with parallel valves but they appear to be angled to the bore.
Photo: http://tinyurl.com/y2t38sor
Quench is possible depending on piston design. The intake and exhaust valves are arranged like the stock GMC head, so it can use a stock cam, many other heads have alternate valve positions and require special cams. There are other GMC heads with open chambers like OEM but 12 port: Fisher for example.

The first commercially successful wedge engines were the 1949 Cadillac and Oldsmobile V8.