Some guys like to downshift and use compression braking along with the wheel brakes. Unless the truck is loaded pretty heavy, and running at a low enough speed that 3rd. gear is actually an option, just stick it in neutral and climb onto the brake pedal. My sports car driving buddies used to get into the non-synchronized 1st. gear in their MG's and Austin-Healeys by locking up the rear brakes momentarily and jamming it into 1st. while the driveline was stopped, but they scattered a few very expensive gearboxes in the process. 6:1 compression on an antique 216 doesn't supply a lot of extra stopping power anyhow!
Jerry