I think Seles and Graf would both probably have the edge prime to prime vs Evert on clay. Graf was just a bad matchup for Evert in general even if she wasnt really a better clay courter. Evert I actually think would have been a bad matchup for Seles, however on clay Seles's persistent and oh so consistent power hitting, accuracy, and angles with power, would have still been a bit too much for Evert most times. I still think Evert deserves recognition as the clay GOAT as she mantained her best level on clay for 15 years, and her day to day consistency on the surface is mind boggling, 125 straight wins over 6 years no matter how weak the clay field was! I do think Graf and Seles both played in a far tougher clay field than Evert did though.
It is interesting to wonder how Lenglen and Connolly would compare on clay at their best transported to the same time as well. Really impossible to guage with any accuracy in their case.