Unfortunately, NT for Alpha only ran in a 32-bit address space.
"The 64-bit versions of Windows NT were originally intended to run on Itanium and DEC Alpha; the latter was used internally at Microsoft during early development of 64-bit Windows. This continued for some time after Microsoft publicly announced that it was cancelling plans to ship 64-bit Windows for Alpha. Because of this, Alpha versions of Windows NT are 32-bit only."
Alpha support was removed in one of the later NT5 betas right? Makes sense that it would've been late 90s then, before it was renamed Windows 2000 for release.
It was canceled essentially overnight by Compaq higher-ups, teams at Microsoft and Compaq learnt when they came to office. It was present on last release candidate before RTM, because it was essentially the only 64bit platform to actually fixup 32bit issues that prevented 64bit address space in earlier NT releases.
"The 64-bit versions of Windows NT were originally intended to run on Itanium and DEC Alpha; the latter was used internally at Microsoft during early development of 64-bit Windows. This continued for some time after Microsoft publicly announced that it was cancelling plans to ship 64-bit Windows for Alpha. Because of this, Alpha versions of Windows NT are 32-bit only."
https://en.wikipedia.org/wiki/Windows_NT#64-bit_platforms