The Savage pillars we sell, mentioned above, fit right into out LRV Thumbholes I might add, no drilling required.
If doing a stock myself I use Brownells 9/16" V-Block ribbed pillar up front for better receiver support. If you don't like the V you can cut that end off instead of the flat end but I don't know why anyone would. They also have some ribbed U-top units but the screw holes are smaller. Also, Darryl Holland has a set of very slick ADL pillars if you're doing a blind mag unit that come with screws (Savage is the same thread) that countersink into them. Haven't tried them but keep threatening to.
Regarding off-center drilling: Suggest using a piloted drill bit and/or drill press if possible! Drift is about unavoidable with normal "Home Depot" bits.
If you are really serious about your pillars get the piloted 9/16' aircraft counterbore version (not a pun LOL)
http://www.brownells.com/.aspx/pid=8011/Product/AIRCRAFT_COUNTERBORE NOT the forstner as the pilot in the Brownells forestner breaks readily, has several arrive already broken. The 5/16" pilot usually fits in most stocks but may have to clean up the existing holes a bit as it may be tight.
Obviously this requires the use of 9/16" pillars but I like them the best especially up front anyway.
Countersink a hole with the above bit about 1/2" in from the top side of the stock (also avoids chipping when the bit exits from the other side), turn the stock over, then take it in from the bottom in about 3/8" deep passes, backing our the bit to clean out the chips. Use an appropriate size block of wood under the comb of the stock (stock is upside down after all) to avoid rocking the stock in the press under pressure and canting the hole. When you meet the top hole you're done straight and true!
Hope this helps,
Don