Since no one replied I’ll take a stab.   My chain is way too short at 20’ so had the same question. A bunch of internet searching had a very few saying that the links are more than strong enough to are you out of you frickin mind.  

The “best” response was a person who cut the end link, attached the new chain and professionally had the cut link TIG welded together, then had the whole thing regalvanized.   Sounds expensive and a lot of work.  

I’ll get 120 feet of new chain and save the other for the backup anchor.  For me it’s not worth the stress of worrying if it will hold

Mahalo

I had 80 ft of chain on the boat when I bought it...  I added 100 ft more and used a 'Acco C-Link' (make sure that they are a ​​heat-treated, carbon steel C-link (Heat-treated links, that are marked with a load standard are virtually as strong as the chain which they join. Provided they will pass through the gypsy, which most will, they are fully acceptable for yachting use).
West Marine makes a product ( ​https://www.westmarine.com/buy/seafit--connecting-links--P005_154_002_004 ) but I'm unaware if these are heat-treated like the Acco product.
I've had it in use for 5 yrs and anchor about 60 - 70 nights a year on the north coast of BC usually in 40 feet depth, sometimes more, in winds sometimes upwards of 30 kts.  No problems so far with my 180 ft chain and Rocna anchor  (fingers remain crossed and knocking on wood)...   :)