Thanks for the help guys. It looks like __atoi64 is the closest thing to what I'm looking for. Unfortunately, it doesn't read numbers in any base other than base 10, which doesn't suit me since I'm coding a compiler engine and must accept binary, octal, and hex as well.
It looks like I'm just going to make 64-bit math a compile-time option for my project, since my goal is 100% ANSI compliance, and the ANSI standard gets a bit fuzzy when you up it to 64-bit (integer-wise).
Thanks again for the help.