I have a date of 1978-1979 when most, if not all, the metal work was replaced although the supports are orginal.
Any further info would be much appreciated.
What sort of thing do you want to know? I can find a couple of newspaper pieces from 1978, though 1979 is off the end of the archive!
Work started on June 12th 1978 on the westbound track at the Neath end, and was due to end at the other end in December, though the river bridge span was retained. All the wrought iron plate girders in the approach spans were replaced with modern steel, each span temporarily replaced by a "helper" for the half still in use.
The eastbound half was due to take from December until April 1979. Note that this was not a case of two separate structures, but the strength was in the girders underneath and half were swapped at a time. You can see now that there are rivetted plates joining the spans together in the middle, so presumably these were put in in situ.
The result was that the weight limit (RA) was increased, and the speed restriction went up from 20 to 40 mph.