On your 250 you won't see the distributor move like the earlier 216/235 distributors. The advance is inside the distributor for your truck, the body of the distributor won't turn when clamped down. You might want to think about checking the fuel pressure and even do a compression test on your engine. After its warmed up take the air cleaner off and look down into the carburetor and then move the throttle, see if you get a good squirt of fuel. If not you either have a fuel supply issue or a bad accelerator pump.