I put a pair on the '53 F100 that I had and it had a stock '66 289 in it. It didn't sound that good to me IMO. It was noticably 'raspy' with a little burble to it. It was, however, fairly quiet at cruising speeds (55-60mph). At WOT it didn't sound as deep & solid as I wanted it to. I sold the truck and the new owner seemed to like it. 
It is a very heavy muffler for it's size.
I believe John 'johnmustang' has a set on his '65 fastback and is very satisfied with it. 
It's different for everyone.
Erik
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1966 Ford Mustang 2+2
Mine - Restomod in Progress
1966 Ford Mustang Coupe
Dad's - Original Unrestored