Well..... one doesn't want anything that is aluminized, at the very least 409 stainless, you might look thru Walker's Quiet-Flow SS series, their recent all stainless lineup may include something close to the original mufflers.
Consider that you may end up going to a pair of 4 1/2x9 3/4" 14"shell mufflers.......the left side IS a 12" shorty while the right should be a 14", the right IS a 7x9 shell.