Thank you all for solving my problem with dry rodent tissue following
processing.  I still put the blocks on ice just prior to sectioning, but not
for nearly as long.  The 70% storage seems to have been the culprit - I now
store in 10% formalin indefinitely, or for IHC tissues I transfer to PBS
after 24 hours (and have the tissue pieces small).
Presently I'm working on optimally processing rodent eye tissue.  I'm fixing
still in 10% NBF.  To keep the lens, or not to keep the lens.  To bisect the
eye or not to bisect.  To decalcify or not to decalcify.  The problem with
the rat eye is that the lens takes up most of the eye volume - it's not
nearly as small (respectively) as the rabbit lens.  I initially removed the
lens but ended up losing the shape of the eye, and the retina detached.  The
same results when I kept the lens intact until just before embedding the
eye.
We're just doing basic H+E right now.
Thanks for your wizardry.