The weight of that BB is of little concern. The overall stopping power of the truck was built into the truck determined by the brake size.
The truck was built to have weight added to it in the bed and be able to stop.
Now if you load the truck to the max, plus the added weight of that BB, then you might over come the brake systems ability to stop that rig safely.
Now weight distribution may be an issue because your adding more weight up front and when you stop the rear becomes lite because of weight transfer which usually causes the rear to lock up and skid around, especially on a slick surface.
I don't own a 57 GMC and it's been to long since I drove one so I can't judge the stopping power it has.
If I felt the truck stopped well now, plus the booster your going to add, I don't think I would need to switch to disc because of adding a few hundred pounds up front.
I have a pickup and I added weight to rear behind the axle to improve traction and help keep the tires on the ground when braking.
Now if you have the formulas, ie. weight vs brake surface area, then this could be figured out on paper.
Keep in mind, these are my thoughts, all of which are not facts.
Don't forget to upgrade the front springs for that added BB weight.
Dan
