Most of the newer vans I've looked under have 70's.
ZPD307, for crying out loud stay away from the GM calipers! They are front calipers used in the rear, the Caddy stuff is from the rear of a 5000 pound CAR. The only people winning from using these parts are the people involved in making and selling the parts to do it. I learned first hand they do not work more than a decade ago when the GM disc swap stuff started to become real popular. I put more effort and money into trying to make those parts work then 10 complete rear drum replacements would have cost me and I ended up tearing them off after I melted the 4th pair of rotors towing and went back to 1 ton rear drums. You know what? The drums worked perfectly and laughed at the same loads and grades that fried the GM rotors/pads. The parts fried because the rate/bias was wrong. No matter how I set up an adjustable proportioning valve, what master cylinder I ran, it didn't matter. The GM discs are a mediocre solution to a problem that doesn't exist. They are an expensive one too. You will spend around $500 on average to do it unless the parts just fall into your lap. You can buy a complete late model rear axle from a wrecker for that, probably get the entire brake system with it. If you're craigslist savvy you can do it for a couple hundred bucks or under.
If you don't feel I'm being accurate paraphrase some of the stuff I've written here and call a vendor for these parts like ORD. Ask them how well front discs in the rear will work on your drum truck compared to stock 1 ton drums. Ask them how the mismatch in master piston area to caliper piston area effects the versatility of a stock, matched braking system and how you can overcome this. Ask them how the residual pressure valve in the rear brake circuit of most rear drum vehicles (in the master cylinder often) effects disc brakes when they have 5 PSI of constant pressure pushing on the caliper pistons when not using the brakes at all. Ask them how an adjustable proportioning valve (pressure regulator) in the rear brake circuit is supposed to be a solution to a mismatch in the rate of fluid transfer from the master piston to the caliper piston in relation to pedal movement?
The answer you will get will be something like "The GM discs in the rear will work well for most types of driving and SHOULD work fine under your truck. If you want everything perfect you should keep your stock brakes."
The most possible bang for the buck you can get for upgrading any stock disc/drum braking system is going to hydroboost. My 11K pound empty 89 F350 4x4 stops like a Miata with 10K behind it (exageration, but not far from reality, it really does brake extremely well) and has 1 ton discs/drums with hydroboost.
