This is my exact thought for my truck also. In no way shape or form would I only leave 1 spring in place. The springs locate the axles and taking the chance of leaving 1 spring leaves the door open for the axle to move side to side.

What I was thinking was to pull every other leaf out and install the bags from there. Also I would go with air bags that have built in bump stops or fabricate bump stops on the truck. Always account for a possible failure.

I have made a few custom suspensions over the years, nothing on a medium duty just smaller vehicles. After having back surgery my back doesn't like an extremely rough ride anymore. I though about a custom 4 link for the rear end also but decided against it. If I'm hauling 20k pounds say though the mountains one day and something doesn't stay together there isn't a failure that can be accounted for with a 4 link that is busted.