As Geezerbiker mentioned, the 7mm-08 Rem is designed to be formed from 308 Win brass without touching the neck or having to trim the finished case length. This avoids forming a "dreaded donut," as would happen when starting with brass that had a taller shoulder. This is confirmed when you look at the SAAMI drawing dimensions from the case head to the start of the shoulder, which is the same for both cartridges as is the shoulder angle and all diameters up to that point.
What that all tells you is that if there is a shoulder problem, you could try resizing first in a 308 die. That will set the base-to-shoulder length. After doing that, the 7mm-08 die is just bringing the neck diameter down and forming the last few hundredths of the shoulder from the bottom end of the 308 Win neck, which should be pretty easy going. The rest of the shoulder should then go untouched. If doing this is causing a problem, I would check for an existing internal donut in case the brass manufacturer screwed up somehow.