Hey gang,
In the reloads I've done so far, I've used CCI #200, Large Rifle Primers.
Would #250 Magnum Large Rifle Primers be safe to use?
You should be fine. CCI 250's are more or less identical to CCI #34s (Military 7.62 primers), which many people use for slamfire protection. I would remove 1 grain of powder from your load then slowly work up. I find that I usually use .3-.5 gr less powder in a cartridge when switching to a CCI250 primer.
I've never used magnum primers, I stick to Winchester Large Rifle in my M1 loads. Never had an issue. Quick google search seems to indicate CCI 200s are fine for M1s but I've never tried it personally so I can't say