re: Need to hear your positive or not experience with a good auto exhaust shop in BR

'07 300C 5.7L HEMI.
I decided to keep it when it came time to buy a new one. I'm well aware of the overwhelming demographic that drives these things. I like it a lot and decided that having done this all my life I wouldn't let a car go that I really liked and I really like this car. I don't care what anyone else thinks. It had 108,000 gentle miles on it when it went down.

A local mechanic changed the oil and put the wrong oil in it and the MDS malfunctioned and caused a piston to fail. Phenomenally the damage was minimal.

An employee who is a mechanic that works with me asked to take a look at it and had the engine out of the car before I knew it. He'd already re-built two diesel engines recently so we went ahead and took it down to the short block and had the bad cylinder re-sleeved and all of them honed. Got all new pistons, rings, lifters, heads, catalytic converters etc and put it back together and it runs great.

The only problem is what appears to be a resonance that sets up when decelerating from around 23 mph. It goes away if you downshift I think because it changes the harmonics of the exhaust due to the change in RPM's.

I've tried some things myself but I nor anyone I know has much experience with exhaust systems other than taking them off and putting them on.

I'm hoping someone with years of experience can tell me what to do other than replace the converters which is costly. I'd glady replace the pipes and mufflers if that would cure it but I don't want to do that unless I'm fairly convinced that will cure the issue.

I want to hear about someone who you know of that you think I can trust. I can give you the name of a good transmission shop and I hope someone can do likewise with a good exhaust shop or mechanic.