Marshall did damn well to reach 1991. The man had heart attacks as far back as the seventies, and considering his lifestyle, making it past eighty was excellent longevity on the bench. You need to handwave quite a bit to keep him going; some deep personal POD, perhaps, like he gives up his two pack a day habit much earlier.
Soz thread, but no, I don't think this guarantees a clearly different SCOTUS. The product of Marshall retiring under Clinton means a vacancy from either White or Blackmun transpiring in 1995; when Bubba Bill was at his most 1994 mid terms contrite-triangulatory, and Congressional Republicans were at their most emboldened.
A fascinating alternate scenario, and one which most likely results in a justice somewhere in the O'Connor mould ballpark, a swing vote. If you want a reliably liberal SCOTUS this late, then you really need a two term 1989-1997 Dem President - which is a big ask.