Here are the 2 engines I'm looking at.
Robert, your engine mounts remind me of the 1918-19 Keystone frames which were the newer style frames back then that allowed the engines to be set lower in the frame.
Motorcycle / Keystone ( Marion Indiana ) Frame
Keystone or Marion Frame: An American term to describe a diamond-type frame in which the engine serves as part of the structure.
Motorcycle / Loop Frame
Loop Frame: Early motorcycle frame design in which the downtube curves underneath the engine case and then upward to become the seat-post. The earliest motorcycle builders were adapting gasoline engines to 19th century bicycle frames which then evolved into sturdier chassis construction to hold larger engines within the frame loop.
See the link for these frames below:
Frames - 8 valve racer
Harley Davidson 1918 Keystone Racer
http://motorbicycling.com/f38/history-original-board-track-racers-18232.html
In 1919, Indian debuted its challenger to the Harley-Davidson IOE (intake over exhaust) racers that had dominated American race tracks since 1915. The new Indian challenger was a side-valve derivative of the famous Powerplus which, after its record-setting speed trials in 1920, became known as the Indian "Daytona." The 1919 Indian racer that appeared at the Marion, Indiana Labor Day race featured a "keystone" style frame; henceforth, Indian's version of this style of chassis was known as a "Marion frame."
Harley-Davidson pioneered the use of the "keystone" frame in their successful 1916 win at the Dodge City, 4th of July 300-mile race.
The keystone frame was actually a modified loop frame with the bottom of the loop cut away. This allowed the engine, supported by plates, to act as a structural member of the chassis. Although Harley-Davidson's success at the 1916 Dodge City race was due to various innovations (8-valve engine and hemispherical combustion chambers, for example), the improved handling characteristics of the lowered engine position prompted Indian to follow suit.
Robert, just something to think about for a future build that is a Keystone / Marion frame style build with the drop loop removed and using the engine mounts to strengthen the frame.
Peace C.H.