This had some info on the horizontal 12, but no photo.
White Motors Heavy Duty Trucks - Diesel Power Magazine
"Following a board decision to return to independence, Robert Fager Black was appointed company head in 1935. Excellent with workers and the union alike, Black slowly began to return White to profitability. 1935 also marked the introduction of the heavy-duty 730, the company's first cabover. This novel platform was powered by a 7.6L (464ci) opposed-piston, horizontal, 12-cylinder engine. The powerplant was a smaller version of an 8.3L version utilized in buses in 1932; tilt-cab, 800-series versions of the truck appeared in 1937.
Read more: http://www.dieselpowermag.com/news/0706dp_white_motors_heavy_duty_trucks/viewall.html#ixzz2g5yZbuSh"
Another interesting article on engines...
The 10 Most Unusual Engines of All Time – Feature – Car and Driver