its only worth what people are willing to pay for it, having mods like that can also go against the value as some people like to do that stuff them selves as its a personal choice.
I had a very similar problem where once everything was warmed up the revs wouldnt go below 1000 rpm when slowing down, it ended up being the torque converter, I cant remember the exact problem but it fixed the issue.
go to a wrecker like pick a part or something like that and learn by taking apart one of the doors there, take some basic tools ( phillips and flat head screwdrivers and a socket kit) you will be surprised what you can learn from doing it your self
It wont be air in the line, recheck the injector connections, check the o-rings as well as they might not be seated correctly and will be sucking air past them, disconnect the injectors while the engine is running to locate which ones are causing the issue, it could also just be a faulty...
A mate of mine did this a few years ago only he used adjustable J pipes and basically spent a whole day driving up and down the freeway adjusting the J pipes till the drone was at a minimum, in the end he got the drone down to 2 or 3 db below the stock rear SS mufflers.
If you can find yourself another 1CVA or another #CVA with low Ks then use that, the only thing you will have to so is use your TCM from your transmission as its coded to your car.
https://fluiddrive.com.au/wp-content/uploads/2017/08/GM-Holden-WallChart-May-2017.pdf
There is a list of...