I used to add weight to my 63 Dodge to slow it down for N/SS. It had plates with 1/2" nuts welded just below the floor under the package shelf area. There was a plate with a 1/2" hole located directly between the nuts. The nuts were spaced far enough apart to fit the diameter of 25lb dumb bell wieghts. A piece of 1 1/2" angle iron was layed across the weight with a 1/2" pin through the middle hole of the weight. The angle iron held the weights down with the 1/2" bolt on each side screwed into the floor nuts. The 1/2" bolts and the pin were long enough that I could stack 8 wieghts side by side. You could stack 16 25lb wieghts right above the axle and change them very quickly. loosen the bolts lift the pin up, slide the required weight in or out, reinstall the pin and tighten the bolts.
Doug