You're very confused.
The 270 and .300 WSM have the same shoulder location. The neck has nothing to do with headspacing. Just neck .300 WSM cases down, and they should work in a .270 WSM. The only potential problem would be the case neck possibly becoming too thick--but that's easily checked with a caliper.
The 7mm WSM's shoulder is slightly further forward than either the .270 or .300 WSM's, to prevent 7mm WSM rounds from chambering in .270 WSMs, where the larger-diameter bullet would result in MUCH higher pressures.
