The helicoil doesn't need anything other than a blind hole to go into, the whole kit is designed to work with it. One thing to note with a helicoil kit, though, is that the cheaper kit does not come with the appropriate drill bit to widen the hole before tapping. And most of the drill bits are not your normal size you'll have in your garage drill index. The helicoil pro kit, which is more expensive, will include the proper drill bits.
If you look at this pdf, page 16, it will list the drill bits needed to drill before doing the tapping. For 5/16-24, you'll need a 21/64 drill bit.
http://www.afi.cc/co...CoilCatalog.pdf
Most blind holes from a factory were originally drilled without a bottoming drill bit or tap, so I've often been able to get by with a standard drill bit and tap. The above PDF also has the depths needed to drill for the different size helicoil units. Sticking a probe down the hole to make sure there's enough room for your helicoil would be a good idea.
