That's strange that RCBS would sell you a die set with an odd diameter. My Lee dies measure 7/8" as do standard RCBS dies. RCBS does make an Ammomaster press that can use standard dies and can be converted to use those 1 1/2" 50BMG dies. You kind of fall right in the middle. What did they think you were going to use your dies on?
Since you plan on loading 50BMG, I would think you'd have to get one of these Ammomasters anyway. See if RCBS can make a conversion sleeve for you.
I can't really think of any brand that'll take 1" dies since all of them seem to be able to take each other's standard dies. Even that new Lee takes standard 7/8-14 dies then jumps to 1 1/4".
I'd give RCBS a call to see what they come up with. Maybe they'll make you a conversion sleeve for free, that'll fit the Ammomaster.