I have found ethanol and isopropanol to be rather similar in thier
dessicating abilities, but in order to get the best benifits from the
latter, it should be 90%+ pure- 70% simply won't do it.  Some drug
stores sell 98% , which is great!  One does not want to use a bleaching
agent such as peroxide, unless just brifly (up to a few hours will not
harm the pigments much), and rinse w ell afterwards.
        Re the problematic slightly larger shells: try adding a few drops of
formalin (use this in a lab, or outside, however), letting it sit  for a
week or so, then rinsing for a few hours in water.  Also, changing the
alchohol a few times helps considerably.
 
        Nobody has any interest in my Turridae posits?  I find this monstrously
large and insanely complex family endlessly fascinating- it seems to
comprise 10-15% of all shell-bearing molluscan fauna in most regions,
but many spp are incredibly difficult to find!!  Of the 40 or so i've
seen from Eastern and arctic Canada, for instance, nearly half of them
are represented by 3 or fewer specimens!
                                                                        -Ross M.