I've got my first-ever Pinewood Derby coming up in a few weeks.
....
I have gone from dreading this project to being "into it" enough that
I've had to remind myself frequently that it is my son's endeavor, not
mine. I look forward to hearing some tips from a few voices of
experience.

You have a couple of replies that mention the nails and nail slots in the
block. Be sure that you know the rules of your pack and district race (if
your district runs a race) regarding these modifications. Our district race
is coming up soon and any car with re-drilled axle slots will not be allowed
to win. (I will never prevent a scout from at least running his car down
the track, since the parent is almost always the reason the car is ruled
ineligible.)

Ken