If the van originally had drum brakes front and rear, I suspect there is some kind of residual pressure valve plumbed to the front and the rear, to keep the shoes in close proximity of the drum inner surface. The 11" brakes will work a little better than the 9" brakes, simply because there is more shoe surface area in contact with the drum surface area, but I'd bet it will be close enough that you'd never have to jump through any hoops to make it work. I'd put the diff into the van with the 11" drums on the rear and run it. You may notice a little better stopping power and no other significant differences. It'll be close, front to rear, because the slave cylinders are the same size.

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