Here's how I went about cutting the hole for the Weldment for both sides. 

First draw a grid as shown.  (This is on the left side.)

Draw a grid on the Template and then line it up on the skin and mark the hole.

After cutting the preliminary hole I realized that this Weldment is longer than the other and won't fit no way no how.

It measures 13 1/16" as compared to the right side that measured 12 5/8".  Vans is sending me a replacement.