Unfortunately you have one of the last PowerStack machines which came with Firmworks OpenFirmware (non-Sun implementation) out of the factory instead Motorola PPCBug. These only will boot AIX (or linux as you said before). Not sure if was possible to make these run NT with firmware change or ARC disk, although some docs pointed a "future" compatibility. NT itself was compatible to some extent with Firmworks OpenFirmware, but the built-in support was limited to the FirePower Powerized workstation series made by FirePower/Canon.
Why? In Openfirmware machines NT needs a shim firmware-on-disk executable which emulates ARC firmware in the OpenFirmware environment called VEENER, similar to what MS does actually to boot Windows on the RPi compatible boards, or to Intel's DUET/Hackintosh EDKII Clover, which can be used to emulate UEFI in a BIOS-only machine. But unlike newest incarnations of that shim technology in newer windows versions, VEENER has to team with NT HAL to boot the system, and HAL has to have support for the VEENER module used to boot the machine. Unfortunately none of the HALs except the one for the Canon systems have such support, so you can't use the VEENER included in the NT CD to boot Motorola OpenFirmware Machines with the built in Motorola HAL, it will BSoD early in boot process. You need an ARC disk with an specific VEENER/HAL tandem tailored for your system, and i'm not sure if such disk ever existed.... That or a firmware disk to downgrade your actual OpenFirmware firmware to PPCBug.
Finally, maybe there is a possibility to create a Veener and HAL tandem for the Powerstack using the leaked windows source... If you know how to do it... Supposedly according some webposts at Betaarchive that source has all the tools and the source code of both Motorola HAL and FirePower Veener/HAL tandem required to build these files, but you still need to port the support for Veener from the Firepower HAL to the Motorola one, and fix the VEENER module a bit 😜.
ED: There is at least one reference to VEENER.exe in this OpenFirmware manual for your system....
http://www.ing.iac.es/~docs/external/vme/PPCOFWQSA-UG2.pdf
You can try to boot NT CD with the commandline "boot cdrom:,\PPC\VENEER.EXE" from the openfirmware prompt and "tell us" what happens 😉.