This is a van converted to Transport wheelchairs.

The brakes can already lock up the wheels and make them skid. You can't do any better than that. All larger brakes will do is require less pedal effort to make them lock up. Larger brakes WILL take longer to get hot in stop and go driving.

Chrysler had a bulletin years ago that only applied to their larger trucks that were constantly fully-loaded resulting in rapid front brake pad wear. You could install one size larger rear wheel cylinders to increase the percentage the rear brakes contributed to stopping. If that wasn't sufficient, you could go two sizes larger, but those wheel cylinders had to come from a GM dealer because at that time Chrysler didn't offer them because they weren't needed on any of their products.

Going up one size on your rear wheel cylinders would be the only practical solution since most of the added weight will be on the rear wheels and they can be asked to do more stopping without fear of easy rear-wheel lock-up. That does affect how fast the shoes will heat up but it will take some of the stress off the front pads. To go to larger shoes and pads is not practical. That would require changing the rear backing plates and drums, and potentially the rear axle that those backing plates fit, larger front calipers and the spindles they bolt to. A lot of development went into selecting the right combination valve in the brake hydraulic system to balance the front and rear braking power and to prevent rear wheel lock-up under hard braking. If the van has anti-lock brakes, the valving and computer timing are all tailored to the original weight and weight distribution of the van. Most trucks and vans already have height-sensing proportioning valves in the rear to adjust that braking split for changing weight conditions. That makes up for adding a bunch of passengers or a load of firewood. Cars have a one-size-fits-all proportioning valve because adding weight isn't such a big factor in changing the performance of the brakes. With trucks and minivans there can be such a big difference in the weight distribution that the designers can't accommodate all the loading possibilities with one preset valve.