We are putting together a 1985 Mustang GT with a new 347, 430hp, and have a question about exhaust. We are looking for something that sounds good but not loud and obnoxious, the owner is in his 60's. Has BBK shorty headers, dual exhaust with H Pipe. Thinking of the flowmaster 50 Delta?
Kind of an apples to oranges comparison, but I have a 50 Delta Flow single in/dual out in my Ram 5.7 Hemi. It now has shorty headers as well, otherwise it's completely stock. I think it's a good compromise between sounding relatively "aggressive" without making you go deaf. So my vote would be for a pair of them as well as a pair of converters as @Noobz347 suggested.
Another option - my bone stock GT has a pair of Summit brand 2 chambers and I believe it's about the same volume as my truck. But again, it's completely stock atm - stock headers, stock converters, stock tail pipes.
Another option - you could also add a pair of glass packs upstream of the mufflers to act as resonators to help quiet it down if you can't quite get to what you're after otherwise. I did that with my Cobra.