I've heard several people complain about having problems with this, like the mount being warped, or the pilot holes not lining up or whatever, but it turned out to not be a big deal, at least in my case.

Loosely clamped the mount to the firewall with a 2x4 and some furniture clamps. I could see the pilot holes through the mount at all 4 corners so far so good. Drilled port upper to 1/4" and put a temp bolt through it, then fine tuned the location.

Marked the two middle lower locations through the mount with a sharpie. Then pulled the mount back off, checked marks for edge distance in underlying structure.

Reamed 1/4" hole up to .370 for AN6 bolt. then reinstalled mount, snugged up that bolt and clamped with furniture clamps etc to really lock down the position. Checked with digital level between spar center section and mount cross member. got them to match within 0.1 degree (tightest tolerance inclinometer will measure).

Drilled all holes with a cobalt drill bit and lots of boelube. That bit was 3/8" but it was obviously slightly undersized. Reamed final size with a .370 reamer for a nice snug fit. Installed bolts as I went. No problem.

I needed a .020 shim under the port side center lower mount, which is addressed in the build instructions, other than that, everything seems nice and flush. I've got to go back and trim the lower flanges where they will interfere with the MLG legs, but for now, this is good enough for locating battery, oil cooler etc.