I've got FlowMaster 50s on 2 cars.
My '62 NY stationwagon, with a stock 413ci engine and 2-1/4" dual exhaust, is very quiet.
But its engine burns some oil and it has deposited the soot mostly in the (some 15 year old) mufflers which toned them down very nicely.

Thinking I could copy the sound into my '65 300 convertible with a 400ci with 2.5" dual exhaust and an 'advanced' cam, and it turned out to be fairly loud.
It has the newest of the bought Flowmasters. Has a nice rumble while driving and idling, but is very loud when cold. Actually a bit unsuitable for the car.

My '60 NY with 496ci engine has Dynomax mufflers but I had to add bullet style resonators at the rear to tone them down a bit. But they are still very noticable.