Yes- right path, but the 300gr. might need a GC as you thought. Not sure how that will happen unless you have a lathe to turn down the base.
If so, you can apply a .44 mag gas check before sizing down, just as with the pistol bullets.
I have sized down .375" 310gr. bullets to 9.3 with excellent results.
Of course, grooves (crimp) that are not filled with lube, get reduced in size.