The Classic Client 6.3.12 remains a robust choice for enthusiasts. While there is no "native" 64-bit version of this specific build, the x86 version runs exceptionally well on modern systems provided you manage the DPI scaling and administrator permissions correctly.
Are you planning to use this client with a specific or on a private server ? classic client 6.3.12 for 64 bits
A 64-bit OS doesn't always come pre-packaged with the legacy libraries that 6.3.12 requires. You may need to manually install: The Classic Client 6