I have NOT lowered an earlier B van, so this may not apply.
I did lower the front of my 2002 B2500. Cut just under a full coil off. Rides poorly, so I expect you will also need to change shocks and of course an alignment. The rear requires longer shackles, BUT the shackle goes from the body UP to the end of the spring, and to lower it much will put the spring eye above the floor, requiring cutting a hole then boxing in the area. I didn't do that yet.
I also swapped on late model truck wheels that required some spacers, studs and grinding in front, and some thick spacers in the rear, with low profile 17s.