rear brake question

You may throw you brake proportioning all out of whack with this big increase in drum diameter and surface area; it might work or it might not; from what I have read, the police option brakes on B-bodies was 11"x3" drums. BUT, the brake size was changed both front and rear with this option to keep the right proportioning.

Impossible to tell without knowing the diameter of wheel cylinders versus your original ones, and what you have up front. What year car is this going on and what was the year of the van? Maybe we can look it up. I am only seeing 12" x3" for 1982 B150 vans; the 11" shows only in 2.5" width. They do show quite a variety of rear wheel cylinder sizs from 7/8" to 1-1/16" diameter for the 12"x3".

Too big a rear brake will cause premature rear brake lockup; that's bad juju on a wet road or even on a dry road at high speed in a panic situation ==> spinout.