I've formed LOTS of Whelen brass from .30-06 in the past. I believe that Hornady still makes a tapered expander unit designed for installation in RCBS dies. The tapered (Hornady calls it eliptical) expander ball will make it much easier to expand the neck of the .30-06 brass to .35 caliber. You won't get as many "case failures" if you get the conversion expander ball offered by Hornady.
However, you can still form brass just fine using your RCBS dies just as they are, although it won't be nearly as smooth as with the tapered exander ball. Simply set up your dies as you would for any other caliber, lube your cases as you normally would, then use a "Q"tip and some STP oil treatment to lightly lubricate the inside of each case neck, (this is very important to uniform necking of the brass), then run through the sizing die as you would normally.
You'll get good cases, but do be sure to square the necks with a trimmer.
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