ok the guy that had my 2000 max before me cut out part of the y pipe, there is no cat and no intermediate pipe. the smart guy just welded in a straight pipe and unpluged the O2 sensor. my exhaust smells really bad, when i go to take off if im not over 3000 rpm then the car will die, sometimes it sounds like there is a backfire in the intake manifold. if i change my whole exhaust and get a high flow cat and the O2 sensor would this solve my problem? here are some pics of my exhaust.....

I think your problem stems from the O2 sensor not connected. The smell concerns me, like a possible leak in missing cat area. The Nissans are touchy with sensor issues. You might start with fixing the y-pipe first, then add your other exhaust mods.

At a minimum you need a bung added for the O2 sensor. I assume you have a constant SES light as well, if not then it's blow or the bulb has been removed as you will be throwing codes all over the place.

install a bung, reinstall the 02 sensor and see if the smell goes away, theres unburnt fuel comming out of the exhaust thats what stinks