they are a matched pair , & they are the first generation of of the non desireable heads used on the 73 340 & 73-76 360 engines , they will definatly make less power then the X or J casting & if you are going to replace them I would suggest the 308 casting for more power than the X & J head. The -8 is to do with the casting process & the number of times the mold has been used to make sand castings , the 4 digit # should be the date cast 10th day 73 & 322nd day 72