Here's a write-up by Prof Fish Heads (PJ Stoops) for preparing oyster drills; he indicates the same general approach works for whelks and other snail-like critters. http://professorfishheads.blogspot.com/ ... mment-form
You might drop by PJ's "Total Catch Market" on Sat mornings (only) at the seafood wholesaler Louisiana Foods on W. 12th off of N. Post Oak just outside the Loop (Houston). Features bycatch from local fishing boats. He loves to talk about preparing unusual seafood, and might have specific suggestions for whelks; he has a particularly affinity for Thai and other SE Asian preparations. http://totalcatchmarket.blogspot.com/
Be sure to share your recipe when you work it out!