I have a pair of used tail light assemblies (I believe they are from a 1979) that would fit any of those year Regals. They are in driver condition. Lenses are in good shape, not cracked. $50 for the pair + shipping.
78 and 79 are different, but will interchange dimensionally. check the tiny lettering on the corner of the lense. it should say SAE and then the year - 78, 79 etc.