That's straight from the manufacturer, and it explicitly states what is required for FMVSS compliance. Deviate at your peril. If the vehicle ever gets inspected, or if a crash situation results in compliance with those requirements being called into question, by the cops or the insurance or the lawyers, you'll be in a heap of trouble with no way to defend it unless you actually have complied with all that.

Calculating where the center of gravity is, when you know or can reasonably estimate the weight and position of everything in the vehicle, is not complicated ... only tedious if there are a lot of bits and pieces to add up.

If it is the extended-length E350 (with the extra piece at the back grafted on, same length as the 15-passenger van) be very cautious about extra weight behind the rear axle, i.e. try not to have any more than necessary back there.

Try not to put anything heavy at ceiling level, that could be at floor level or below. Common sense ... but some people need it spelled out. Try to put water tanks and the like, below the floor. Likewise with batteries.

If they are grafting on a high-roof extension (so as to be able to stand up inside), doubly so.

Thanks, and for reference, this is the 99 MY guidance, I put up what I found first, 2003 MY information.
https://www.fleet.ford.com/truckbbas/non-html/1999...