How to convert a string to long int?
If all those are known tags, you can put them in the source code as
32&nbsp;bit integers. No
need to convert. Or rather, let the compiler do the conversion:

const uint32_t ID_num[ARRAYSIZE] PROGMEM = {
    Oxbcc0f1c3, Ox821a7d39, Ox4924887c, ...
};


When you read a tag number in hex, in order to compare it with the known
tags, you convert it to the same type using strtoul() from
avr-libc:

uint32_t tag_id = strtoul(tag_string, NULL, 10);