For Montana's first SB the 49ers had no running attack (Earl Cooper, Ricky Patton and Lenvil Elliot [Paul Hofer was tragically injured and had great talent]) which in part was why Walsh implemented the West Coast Offense (not saying he invented it); Montana's wideouts (Solomon, Clark) were decent but not great players. Also defenses could still knock the snot out of you without penalties.
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Bill Walsh DID invent the West Coast offense. He did it in Cincinnati tho, not in SF.
Clark was a great player. He wasn't fast, but he was excellent.