For the centerflow hoppers, you best bet is to get a scale to weigh the car. After adding the trucks and couplers, find out how much additional weight you need to meet the NMRA recommended weight of 5oz plus 1oz for each inch length of carbody. The car is 13" long so it comes out to 18oz or 1lb 2oz.
Since the car bottom sags, is flimsy and not sturdy, i would find a flat piece of steel stock from a big box store, cut to size and glue it across all inside hopper "v crest" with J.B. Weld. This will sturdy the frame as well as add the needed weight to the car.