For NGC 5972 with z = 0.03, the OIII 501 main line should be redshifted by about 15nm. That's also the center wavelength used in the study (see here http://www.stsci.edu/hst/phase2-public/12525.pro). Besides a green filter, a comet or Swan band filter (width 25nm) could as well be useful. Which center wavelength and bandwidth does the Sloan G filter have?