I am using the original speedo on the 36 with a 518 trans. The rear is a B body 8 3/4 with 3.55 gears.
31 tooth speedo pinion. It works well within about two miles per hour. I would test the cable by removing from the trans and turning with a drill. That would isolate which end of the cable the problem is. Also check the squareness of the cable ends and their matching drive holes. The cable could be skipping in the square hole.