you probably already know this but a larger pipe can actually not flow as well as a slightly smaller pipe. it is a science to figure out the pulses and the associated slight vacuum created behind each one. if the pipe is smaller the flow in the pipe is faster so the vacuum pulses created help to scavenge the exhaust from the cylinders as well as help pull the next pulse along the pipe.
anyway, a longer muffler would help, a chanmbered muffler would help, a resonator would help, running the pipes out the rear would help. I have driven the family van long enough (years ago) to know I am not a loud exhaust guy. I am using a 2004 GMC Envoy frame and Atlas inline 4.2L inline 6 cyl engine. I will be keeping the stock exhaust because, firstly, it's already in place and quiet, secondly, I'm cheap. lol.
https://www.motortrend.com/how-to/wh...pes-work-best/