So, a bit of an update. Marky Mark gave me a hell of a deal on a nearly new set of Redding 378 dies, he is such a good guy I actually had to force him to take money for them. Thanks Marky Mark, they worked great. Here is the result of the resizing operation.
From right to left, 378 brass that was necked up to 460 originally, then for the current purposes necked back down in the 378 seating die with the plug removed, then the FL 378 dies with the decapping gear removed, then partially sized the 338-378 seating die with the seating stem removed just for illustration purposes, you don't really need to 2 step this part, then fully sized in the seating die, and finally the FL 338-378 FL die with the decapping rod and expander button in place. Imperial sizing lube applied liberally goes a long ways to making this easier.
A Guy could probably get away just using the two sizing dies but this is smoother and easier. Brass thickness went from .013 to .015-6. Final outside neck diameter before seating a bullet is .365, so .001 under max. without having to trim the necks. This may seem like a lot of work but when once fired 338-378 brass is $4 each it is worth the effort. New brass is nearly $6 a pop.