By "valve body" you are referring to the flow back valve in the bottom of the pump?
Remove the existing one from your Silca pump and measure the threads. Count threads per inch (yeah, you only have about 3/8" of threads, but extrapolate). Use verniers to measure for taper (indicator of pipe threads).
Compare measurements to thread standard (metric, UNF/UNC, American Standard Pipe, etc.).
Another method would be to remove your existing hose barb from the pump body and carry the pump into the nearest Lowes, Menards, Home Depot and see what thread size fits the tapped hole BY HAND.