I ran my car for over 10 yrs with no exhaust donut and had no problems.
My brilliant ex. guy Tim came up with the simplest solution imaginable.
I wanted to run a full 3" exhaust with no neck down at the swing valve.
He said no problem and found a flange that would accept the 3" pipe.
He welded the pipe to the flange with some pipe sticking into the SV
The flange was the proper distance from the SV to use OEM fasteners.
I forgot all about this until I pulled the Engine a while back to refresh.
There was a witness mark on the SV where the 3" pipe was touching.
My Engine mounts were urethane filled so my Engine didn't rock much.
Still, the witness marks indicated just how much the Engine did rock.
There never was any hint of exhaust leak/noise and I ran full exhaust.
Thanks
Randy