In order to determine an appropriate location for the air conditioner hose penetrations through the fire wall, installed the Engine Mount.
After drilling the left top attachment bolt and installing it, the top right bolt was not centered in the hole of the engine mount. After trying various ways to get the hole centered without success, I used an alignment tool to center the hole at the bottom left, and then drilled the top right hole thru the engine mount even though offset from the existing smaller hole in the firewall. Installed the top right bolt. Drilled the bottom center left bolt hole, which was also not centered on the existing hole, and fastened the bolt. Then drilled to bottom left hole which was centered and fastened it. Moved to the right bottom bolt which was offset from the original hole and drilled/fastened it. Finally drilled the offset hole in the bottom center right point and bolted it.
Based on photos of another RV-10 with its engine installed, along with the diagrams in the firewall forward drawings, determined the best locations for the A/C hose penetraitions to be at the bottom corners of the firewall just above the bottom engine mount braces and inside the row of rivets, which place the left penetration about 1 - 2 inches under the oil cooler plenum box.
Marked the location of the left penetration for drilling later.