Ted, if you're referring to the "original series" X450 (not the 'a' or 'e' models), this won't be possible. They aren't supported in any EXOS version past 12.4.x. Support for the X460 series began in 12.5.1.
There is no overlap in software versions between these two platforms, which is required for stacking.
If you have an X450a or X450e switch, then you can load the latest version of 15.3.x where it will be possible to stack.