Pakistan need a win to stay in the tournament and have never achieved a target higher than the 348 they achieved against Australia in Lahore last year.
Ravindra and Devon Conway scored 66 runs in the first power-play, before Conway fell victim to a short-pitched ball from Hasan Ali after making 39 runs in 35 balls with the help of six fours.
Williamson, who was recovering from knee surgery just before the World Cup, had played just one match against Bangladesh before fracturing his thumb.
But there was no change in the regular captain of New Zealand and he added 180 runs for the second wicket with Ravindra.
Williamson was out at long off off the bowling of Iftikhar Ahmed at the score of 248, while an over later, Ravindra also missed a high shot and was caught at the boundary off the bowling of Mohammad Wasim.