I'd probably use the thicker primer myself, especially since the creed rifles are usually set up for lr primers. You could, however try a starting load or two with the 400 so see how it acts, but I prolly wouldn't push them hard.
In my Lapua 6.5 brass, I use the 450's exclusively. I know that you are short on them currently, but I really do believe that they give more consistent ignition. While I cannot point directly to the research (probably available on the Lapua website?), I know the thrust of their argument was that through their testing, the most consistent results came from #1 - utilization of a small primer, #2 - brass that was designed with a smaller than normal flash hole, which #3 - really benefitted/required the use of a Magnum small primer.
I guess that I just relied on their research (probably lazy on my part) and never questioned the results. I have had great to phenomenal results by using the 450's in my Lapua, as doing so got me results as advertised.
I cannot speak to whether or not a Non-magnum primer would suffice, but I wonder if with all variables involved - undersized flash hole, etc - if you might induce hang fires or other problematic ignition issues. Even if the hang-fire was imperceptible to the shooter, say only lasting a few milliseconds longer than normal, I believe that your ignition/initial pressure/final chamber pressures could be negatively impacted and therefore, overall accuracy and consistency could go out the window.
If my groups and overall performance repeatability were to go haywire, I'd really be questioning why I am trying to load "precision rounds" with less-than ideal components.
I am taking all of this in light of the fact that I know how bad it can suck to be low on required components, so I am non knocking you for asking the question. I have considered making compromises myself in different areas of reloading due to these exact same reasons.
Just my two cents, but the 450 genuinely seems to be the "best" answer for this brass/caliber.
I've got a few hundred rounds with the CCI 400. As above, cups are thinner, but I haven't had any issues even when pushing them hard in my big horn origin with H4350, IMR4451, IMR4831. ES and SD numbers have been acceptable with these combos. In a non-custom action with a larger firing pin you may not be as successful and have cratering at lower than normal loads. Lots of things may impact your success, powder choice, temperature, load, etc. YMMV. If you decide to use them, start low and work up a new load from scratch.
I recently scored some Federal GMM AR small rifle primers and have been using them.