A 16 lb. cap is the spec. given in the FSM. Keep in mind that this is for stock engines, with new (not 50 year old) factory components. Engine mods and/or old radiators may make you rethink your needs.
This question arose from wanting to put an accurate reproduction 16 lb. cap on my son's '70 Sport Satellite from AMD. They are out-of-stock, but they do have the same large rivet type in 14 lb. So, I was going to take the 16 lb. from my '70 Duster 340 and put it on the Satellite. Then, buy the new 14 lb. (if it's factory correct) and put it on the Duster.