No the spar doesn't seem to go to the end of each slot.
Just mark the centerline of spar nad make sure it stays at wing
joint. I don't think it is worth the time or risk to lighten the spar.
The plane really will fly light anyway. It's a real floater. Be sure
to use a couple of 2mm thk. washers to give right thrust on your
engine. And DON'T hook the tail wheel to rudder. The bottom of
rudder is real weak. I took the supplied tailwheel metal and made
a simple tail skid that mounts to the rear of fuse. It will still turn
very well. Get some yellow Monocote trim sheet to go over top of
fuse because the paint will come off on your hand. I cut a strip
about 1 1/8" wide and laid over top. Also did in black too over
canopy area -- but it needs to be a little wider.

Once plane is done -- watch where you grab the fuse sides since
it doesn't take much pressure to break thru the shelf paper. A
friend just did that last week at the field while cleaning.