The 6.5 Gibbs has the shoulder moved foreward to give it about 10 gr of powder capacity over the 6.5/06. Mine shoots best with a 140 at 3200 fps.
A pic of a 6.5 gibbs beside a standard 270 win
I don't think sierra offers the 155 smk anymore, at least I haven't found any.
but the berger 140 with a BC of .64 is a great bullet, as are the 139 gr. lapua scenars (BC .615)
OH BTW you'd want an 8 twist, mines a kreiger 1/8 29.5" finished length, a pic