Hi Kevin,
A few responses to your responses.
Yes, ethanol is generally a better storage solution than isopropanol.
 However, in practice it is usually not used, mainly for the reason
you mentioned - expense.  Not only is pure ethanol heavily taxed, but
in many states it is unattainable in quantity without a medical or
research license, because it is drinkable.  Therefore, isopropanol
seems to be the most readily available, most economical, and most
generally suitable substitute for long-term storage of most
specimens.  I'm surprised that your institution uses 40% isopropanol.
 I suppose 40% might be adequate for maintainence if the specimens
were formalin fixed first.  The institutional collections I have had
contact with (not specifically molluscan, but general zoological or
anatomical) have always used 70% isopropyl.  I would be concerned
that some microorganisms, especially some fungi, might be able to
propagate in a 40% solution.  Also, water-soluble substances like
carbohydrates, glycogen, etc. would not be stabilized in 40% alcohol.
 They would leach out of the tissue over time.  Of course, if the
specimen is just for museum display, that wouldn't really matter.
Still, dehydration and protein denaturation are the means by which
alcohol preserves, and I'd be nervous about a 40% solution providing
a sufficient degree of either effect.  But, can't argue with success
- if it works, that's the best evidence.
You also mentioned that 70% isopropanol causes specimens to shrink.
That is certainly true if you place the fresh specimen directly into
alcohol.  The shrinkage is due to rapid extraction of water from the
tissues, causing the fragile cell membranes to collapse.  However,
specimens pre-fixed in formalin won't shrink noticeably in 70%
alcohol because the formalin makes the cell membranes and other
tissue structures rigid.
I was unclear in my comments about EM (electron microscopy).
Ordinary histological fixatives are often acceptable for SEM
(scanning EM), which examines external structures at relatively low
magnification.  It is for TEM (transmission EM), looking at cellular
untrastructure at high magnifications, that specialized fixatives are
a must.
Regards,
Paul M.