I'm currently working on MythTV 0.24.1 for 64 bit, and there are a great deal differences in the naming of packages between 32 and 64 bit - Myth 64 bit spec will not work without a great deal work on Myth 32 bit,
it's primarily the major number that differs and I see there has been quite a cleanup on the naming in the 64 repo,
a workaround to have the same specfile for both 32 and 64 version is to use if statements to handle the differences in naming,
cheers,
MBantz