Well you can't get a Lee Collet Neck Sizer for the 338RUM unless you send off to Lee to make you one. You can get the Redding Body Die. If you wanted to wait then the Lee Collet Neck Sizer and Redding Body Die will do an excellent job of resizing with ease and very little runout.
Another option is the Redding Competition Bushing Neck Sizer (kinda expensive at $110.00 not including bushings)
MidwayUSA - Redding Competition Bushing Neck Sizer Die 338 Remington Ultra Magnum
and the Redding Body Die. The thing is that the bushing sizers size the outside of the neck to the bushing size and so if you have variances in neck thickness, those variances will be pushed to the inside of the neck where they bear upon your bullet and effect bullet grip, seating pressure and thus bullet runout. IMO if you use a bushing die you need to outside neck turn unless you are lucky enough to purchase brass with close to perfect neck thickness (lots of luck on that one).
Last option is to use the RCBS FL sizer you have and just remove the expander ball. In order to do that you will need to outside neck turn to keep your bullet grip under control. For instance if you size a piece of brass in your FL die without the expander and the outside diameter of the neck is .359" (to pick a likely candidate) then you could outside neck turn your brass down to .012" per side and have a bullet grip of .003"
.338"(caliber)+.012"+.012(both sides of neck)=.362"-.003"(bullet grip)=.359"
Since most factory brass is (Remington, I just measured some) .013" to .014" then you would need to do a complete turn down .012"
but you could pay for a good outside neck hand turner with the money you would save on the bushing die and Redding Body Die.
Seaters do make a difference and I use a Redding Competition Seater for my 338RUM. Less runout and more consistant seating depth.
1. Send off for a Lee Collet Neck Sizer, buy a Redding Body Die and Redding Competition Seater
2. Buy a Redding Bushing Neck Sizer, a Redding Body Die, Redding Competition Seater and a Forster hand held outside neck turner
3. Use your FL die without the expander, buy a Redding Competition Seater and a Forster hand held outside neck turner