It would help if you supplied this information.
It is an E350 chassis, correct?
What is the spec'd over-all length of the rig?
What brand and size tires are on the rig?
What is the weight on each axle during a trip, including what you are towing?
The weight of each axle during your trips will determine everything most accurately. Many truck stops have weigh scales. For $10 to $12, you can weigh each axle. It will reveal where you can make some changes to your tire pressure and springs. Changing your shocks to Koni-FSD will help, but only so much can be expected from that change alone.
Our 2007 E350 front axle is lightly loaded on our trips, weighing only 3260 which includes us sitting in the front seats. With that information, I was able to change our original 4600 pound front coil springs to one rating lower. I was surprised how easy it was to do.
CLICK HERE to read about it which includes before and after pictures. Before going anywhere with our brand new tires, I first need to get a front wheel alignment.
I just bought new tires this past week, Michelin Agilis CrossClimate 225/75-R16 115R. According to the chart for our specific tire (brand and size) we are supposed to have only 40psi in our front tires. It seems quite low, but I will start there. I anticipate even more improvement in our comfort. Our rear axle weighs just over the limit of 7800 pounds which naturally dampens an otherwise rough ride in back. According to the chart, we require 60psi in our rear tires. One thing certain, more air than needed makes a rougher ride than necessary. So keep this in-mind. This is why knowing the actual weight on each axle is such important information to have.
Linear Mode
