This is how to do a big brake upgrade for Generation I vans, the same maybe able to be applied to a Generation II van but as I haven't done it (yet) I don't know.
If your van is over 10 years old I would highly recommend you change the brake hoses, either new stock rubber hoses or braided hoses for your chosen application will do.
Part numbers for the big brake upgrade
Front Calipers - Cardone Rebuilds - Part numbers: 184122 & 184123. These are found on dozens of Chevrolet trucks from '79 - '86.
Front disc pads are Wagner - Part number: SX 153
The rear brakes consist of the backing plates and all related hardware from the mid 90's full size cars, beware not all full size cars have the 11x2 brakes so use a tape measure! A couple of cars to look at are the Chevrolet Caprice Sedan, Station Wagon and Buick Roadmasters. It might also be worth while looking at the big Oldsmobiles from the 80's.
The 11x2 shoes are Wagner - Part number: PAB 462.
The wheel cylinders are Wagner and you have an option of using either 7/8" or 1".
It all depends on how much weight you cary in your van, a Buick Sedan uses the 7/8" and the Station Wagon uses the 1".
1" cylinder - Part number: 51088 for both sides, unfortunately I don't know the part number for the 7/8" cylinder but I'm sure a quick search would fix that