Given the restrictionsattached toyour question, I would suggest you post it on Accuratereloading.com under Big Bores. Be sure to mention what you already have so genuine advise can be given. I'd skip the mention of the .223 alternative though.

I would say that you have identified the "slots" so to speak. Either 416 or 458.
Note, I believe only the Rigbys and Dakotas lack the "belt" that you dislike so much.

For a died-in-the-wool .270 fan you are coming along nicely.

I'm sure you will find much support for the .223 project at this site.