This is all because 375 win brass is almost impossible to get here!
From what I have gathered do google searches; using a lee universal decapper die body, but using a 33 WCF, a 35 Whelan and a 375H&H expander in the die might be the way to gently open up a 30-30 case as they all have gentle shoulder and therefore I assume a more gradual taper on the expander. The second two would have to be trimmed shorter however!
The case should be annealed prior to forming.
After expanding the 30-30 case up to 338-358-375 tapered, the brass could then be full length sized using a 375 FL die.
Is this on the right track?
The reason I don't want to fire form is; if a vertical shot is required for even expansion. The range at which I shot will ban me if I start taking vertical shots, even if there is no bullet and its for fire forming!