The only method I can think of is to search the parish registers for Trevethin and surrounding parishes. Registers for Trevethin go back to 1651, surrounding parishes are Aberystruth, Llanwenarth, Llanover, Goytre, Mamhilad, Llanvihangel-Pontymoil, Panteg, Mynyddislwyn and Llanhilleth. All of them cover the period you are interested in.
The registers are probably in Gwent Record Office and copies may be in the National Library of Wales in Aberystwith.
It may be worth searching the IGI, if you have not already done so, and FreeREG will one day be providing an on-line index to registers but this project, (a sister of FreeBMD and FreeCEN), is still in its early days.
Viv
Mon 1871 co-ordinator for FreeCEN
Free census indexes for all
http://freecen.rootsweb.com/