old thread worth reviving
you could rebarrel a 243 to a 358 Savage 99 action, and use the same carrier, by carefully grinding the shell slots in each carrier slot with a dremel and tapered stone, and checking with a full length 358 shell, until the shell mated to the carrier correctly- then check in the empty receiver of the gun (stripped down, no bolt, trigger) to make sure you can load 5 shells and spin the carrier freely.
it's not hard and I changed a carrier over this way in about one hour of careful grinding or less.
the cutoff and shell guide should be the same, being the 243 and 358 are same family, but you "may" have to grind the right side shell guide slightly if the shell hangs up on the shoulder- it probably will not though