I found some Magnaflow mufflers that look close. Note that they are 4" tall, so they will hang below the chassis a bit. Also note (in case there are some lurkers with other than our late 427 model), that early (pre-731) 427s, FIAs and Slabsides only have room for a 14" long muffler.
Universal design
XL Series
I don't know the performance difference between those designs, but the second one is available in a longer case. I'm also assuming that the oval XL series can have the flow in either direction.