Instead of dowels, I prefer using hardwood stock and a hole saw. You get to choose the grain direction you want, and the hole saw
nicely makes the donuts with a center hole for the bolt.

When you attach this way, I strongly recommend that you use long through bolts to blind nuts in the firewall. Doing so puts
the wood under compression, where it is incredibly strong. If you try bolting the way DA Aluminum standoffs are (one bolt
from the back forward and another from the front back) it won't be as strong and there will be more vibration.