Hi Marlo,
 
Either isopropyl alcohol (= isopropanol) or ethyl alcohol (= ethanol)
can be used as a dehydrant/dessicant with equal success.  I use
ethanol (about 3-4 gallons a week) here in the tissue laboratory
where I work, for dehydration of tissue samples prior to histological
studies.  Ethanol is the most commonly used dehydrant in this kind of
work; however some laboratories do prefer isopropanol, and I myself
have substituted isopropanol on occasion, when ethanol was out of
stock.  Ethanol has a greater affinity for water, and a somewhat
smaller molecular size, so theoretically it may penetrate and
dehydrate tissues somewhat faster than isopropanol, but the
differences are not major.  Otherwise, they should work equally well.
 Either can be purchased in a pharmacy as "rubbing alcohol".  Read the
label - some rubbing alcohols are made from ethanol, some from
isopropanol.  For dessicating samples, the concentration of alcohol
is more important than the specific alcohol used.  You mentioned
using 70-75% alcohols.  These do a pretty good job, but of course
they will not completely dessicate a sample, since they contain
25-30% water (which subsequently evaporates when you dry the shells).
 However, dessication would be more thorough, and drying faster, if
you used a stronger concentration of alcohol.  90% isopropyl alcohol
is often available in the pharmacy.
The hydrogen peroxide recommended by Jim and Bobbi should do a good
job of deodorizing, but not in the same way as alcohols.  Peroxide
works more like bleach.  They are both in a category of chemicals
known as strong oxidants.  They can destroy odoriferous compounds,
and even dissolve soft tissues of a mollusk.  But they do not
dessicate, since they are aqueous solutions (almost 100% water).
Alcohols on the other hand, dessicate tissues, but will not dissolve
them no matter how strong the solution or how long the exposure.
 
Paul M.