To your question - I think the Crosby connector looks fine and the best solution I have seen for connecting chain. it doesn't look galvanized so it will likely rust and need to be replaced after a season or two.
Some questions as you make your decision:
1. Where are you going? We have never needed that much chain in 12 years on the east coast or in the Bahamas. We do well with 170 feet of chain and some rode..
2. Will the connector go thru your windlass?
3. Will the windlass do both chain and rode?
4. What is your other anchor? If you lose the Roca what is "Plan B"? anchors get trapped and you can't get them up, or you have to abandon them in a hurry some times.
The 420 is a big boat but the chain is another 100# s on the bow. If your second anchor isn't fairly robust and a good all round "Plan B", I would put my weight and $$$ the second anchor system and just splice in a 100 feet of rode on the Roca chain.
For what it is worth - On Delicia, an IP 40, we have two 55 Delta anchors rigged on the bow. One with 170' of 5/16 g4 and 130' of rode - the primary & one with 50' of G4 with 250' of rode, "Plan B" We have used them both at the same time, but not very often. Getting two anchors down is a pain, but getting them up is a real pain, and very hard and slow if you need to move in a storm or the middle of the night.
My 2 cents
George
