If the 318 had a 904 then yes you will need a shaft with a 727/4sp front yoke. With your selection with the right front yoke butted all the way into the trans I would check that you have 3/4-7/8" end play with the rear suspension loaded & the car reasonably level as tolerances can be all over the place. You might check & see what it would cost out there to have a longer shaft cut down & balanced if you go that route & if so you'd butt the shaft all the way forward/measure from the center of front yoke ujoint hole back to the forward flat edge of pinion flange (where the straps bolt to)/subtract 3/4-7/8"/give that dimention to the shop. when you get it back move the front yoke back & forth in both planes & if it ain't smooth holler back