Well I think that using all the info provided, I have come to an answer to my original question. The left and right motor mount bosses are offset (front to rear) by the width of one connecting rod, using Nathan's photo we see that this is about 1 inch. It is also noted that the motor mount itself is offset...when mounted on the right side of the engine the isolator block shifts forward, vice versa on the left side. Ford designed these mounts very well...one part number fits both sides even though the position on the engine block is different. I have included a pic of the two mounts side by side, the green lines show how the mounts are offset when the mounting tabs are in line.
My initial question however related to the distance between the bolts that mount the engine to the chassis. If the bosses in the block are 13 inches apart then the distance between the chassis mounting bolts is 13 + 2 + 2 or 17 inches. In the pic of the single mount, there are two lines shown...the distance between them is 2 inches, that's where the 2's came from. Thanks to all for your inputs!