i've had a few based on various improved 30/30 cases, currently shoot a .25x30-30 ackely Improved, fine shooting round. except for some minor differences in neck length, it is pretty much the same as the 25/35 Ackley Improved.
Had a 7x30 improved at one point, same concept. 7x30 brass is a tad beefier, though i have had no issues with case life in using standard 30/30 brass. Since i had no problems, never could justify paying 3X the price for 7x30 brass to reform.
If your goal is hell-for-strong cases, you can reform .375win, long as you dont mind a bit of case forming and neck turning.
I used some .375's to feed my 30/30ackley Imp carbine, but never had issues with case life in any properly reamed 30/30-improved based wildcats. Then again, all my loadings were for keeping things in the contender-safe pressure levels. And the "improved" form does cut back on case stretch.
If you intend to run this in something stronger than a contender, and dont mind the work, .375win brass would be some stout stuff.
6.5jdj would get you about the same results, .225win brass is easy to come by, plus easier to find dies, no neck turning, etc ...this going into a platform like the Ruger #1/3?
