Did a search and found this; does anyone know the exact differences between the early, ('86-'89), and late, ('90-'91), M116 engine in the 420 SEL?

I have a '91 model that needs a heart transplant and have heard varying info on the differences and subsequently the PIA factor of using an earlier long block.

Obviously the ideal would be to locate a healthy late motor, but all I am finding is early ones. TIA for any help.