|"why wont it work"|
Because NT 3.1 checks the CPU and will only run on certain ones. The reason is so that it could ensure that the processor was powerful enough; unfortunately the designers didn't consider the possibility of more powerful processors. It is not unusual for old OSs to struggle with modern hardware.
"i know what i am doing. the ultimate aim - to fool the NT install by making it think its an processor of that times. but idk how to do it"
I appreciate that you enjoy a challenge, but you are asking someone else to solve the challenge for you! The linked article explains the switch to the VirtualBox command to alter the CPUID presented to the guest OS.