All the things mentioned will make the suspension stronger and/or last longer.
But even if you put a semi truck suspension under it, it will still only have a 1350 lb or so payload, because that was the way it was built.
IN the end, all the what if's and such won't matter, legally, it is what it is.
The ONLY way to increase those numbers legally, is to build it, then have it recertified by an independent activity...and trust me, you could buy a couple of loaded 3500 dually's for what that would cost.
I looked at doing this to try and get the truck in my avatar, which started life as a Ramcharger, titled as a pickup truck (since in VA it is considered a station wagon since a load can not be stacked higher than the top of the cab when stock).....the cost was over 6 figures, and that was just an estimate.