The bolt holes are different between the two, the YJ being further apart. The front of the frame on a YJ is right at 0.0625" taller.
So the YJ bumper will fit on a CJ, if the mounting holes are redrilled.
When the bolts are tightend, the bumper will compress approximately the difference in frame height. Unless a small spacer is used.
Bumper length is the same.
1957 WILLYS pickup,
1973 J 4000,
1978 CJ7 DD.
1979 CJ7 360, TH400/Quadratrac.
1980 CJ5 trail Jeep.
1989 YJ the CJ to YJ.