.z64 and .v64 are different formats - slightly. You might also encounter .n64.
.v64 files are byteswapped while z64 files are not. .n64 is a slightly older format which is... let's just say different.
.rom isn't an proper n64 rom extension. It most likely signifies that the type of ROM is unknown. There used to be an ap called Tool64 for doing stuff but you're best of hitting the torrent sites in search of a .z64 version.