Personally, I would definitely pay attention to the numbers. Since you are considering G70 chain, I would assume that you want a lightweight setup that has the strength for extreme conditions. Numbers published by manufacturers are not perfect but they are a good start. And yes, I have seen a Crosby shackle break (not in anchoring service
but they are not indestructible).
The alloy Crosby comes close for G70 and works for G40 chain. If you are going G70, you need extra large end links. Make sure that this is done by someone who knows what they are doing such as the company that you buy the chain from. ACCO used to do this and probably still will. The grade and temper of the steel
are important in the end link, you don't want to create a new weak point. G70 is strong stuff, I remember S/V Jedi reporting that he had bent a 5/8" shackle with his 3/8" chain (I hope that these numbers are right).