I am not sure what your working with or how you could end up with a different sized neck from top to bottom. You do not even need to bump the shoulder on fresh brass (I use a hornady neck die) I even used a 338 win die and just stopped short for my first couple hundred cases. The core is the only part being used to expand the mouth, if you continue the case up into the die it will then show polishing on the neck where the die pressed on the outside: adjust your die down to "polish" the neck but not far enough to touch the shoulder, you can even just run it 3/4 down the neck (or not at all) it doesn't matter..................after firing in your chamber, adjust your full length die in so it just bumps the shoulder (I do not even bump the shoulder) or just neck size with a standard 338 neck die. When and IF you full length size (I do not) just pull the guts or core out of your 375 die and run it threw as if it was 375 brass, This will resize the whole case without disturbing your 338 neck. You may need to take a measurement of your neck at top and bottom and see if we can explain this better as I can not imagine how that is even possible. The neck would be spread by the expander and then compressed by the top of the die and then backed out over the expander and should not have any noticeable taper.