I found a web page that lists two immigrant famlies of Lewellens on the ship "Bark Byron" that left Cardiff, Wales and landed in the port of New York on 5 may 1842. They were : John Lewellen 34, and Rebecka Lewellen 27 Their calculated birth years are thus 1808, and 1815.
Ebeneser Lewellen 48, and Gomer Lewellen 12 Their calculated birth years are thus 1794, and 1830.
We were able to follow Gomer Lewellen (he marries Leanna Rabin on 29 March 1856 in Hamilton, Ohio) in the US Census records as follows: Age 1850 Census 20 born abt 1830 1870 Census 38 born abt 1832 1880 Census 45 born abt 1835
Birthplace Self, Father, Mother 1850 Census Wales 1870 Census England 1880 Census Wales, Wales, Wales
Residence 1850 Census Mahoning, Montour, Pennsylvania 1870 Census Brooklyn, Cuyahoga, Ohio 1880 Census St Louis, St Louis, Missouri
His children are: Dora (1859), William (1861), James F. (1862), George W. (1864), Edward(1869), Alice (1873), Jennie (1877), Nellie (1883), and Charles (1885).
Leanna, William, George, Edward, Nellie, and Charles end up in Colorado although George in 1900 or 1901 moves via Texas to Bryan County Oklahoma where his brother James F Lewallen has a family.