Thanks. I found a passenger list for the ship 'SIDON' arriving in New York on 14 July which has both Edward and Cath Watkins, as well as a party comprising David Jones, Mrs Jones, sons Edward and David, and also a Mary Watkins with them, who appears to be another the daughter of Ed and Cath Watkins. Furthermore, Mary is listed in Edward's letter about emigration. I also found a number of the other names in the letter aboard other ships, all arriving c.July/August 1863 in New York from Liverpool
HOWEVER - what I am struggling with is that the Watkins / Jones party could have arrived by 14 July in New York when Edward signed his letter 7 July whilst still in Wales. I gather it took about 8 days or more for a steamer to reach New York, so unless he postdated his letter, surely he couldn't have got there that fast. Also he's described as a farmer on the ship's list whilst on the census he's a collier.
But the precise match in names and ages for the Jones party seems compelling.
STILL no luck finding any of them in 1861