The 6X5 is just a 6X4 with a large octal base. While they are the same electrically, they are almost never interchangeable with 6X4 due to their size.
Substitutes: 6X5GT, 6X5GTA, 6X5WGT
This is where things get very confusing. There is a popular Chinese tube called the 6Z4. There is also a large American 6Z4 reciter which is in no way related to the Chinese 6Z4. When shopping it should be very obvious which one is which is the American version is an old 6 pin tube and is quite large.
The 6Z4 is simply a 6X4 with a different pinout, so you can run a 6X4 as a 6Z4 but only with the appropriate adapter.