Good question Ron. I know the early 60's had a smaller spline in the disc. BUT, I keep remembering when I was a teenager, (that was a couple of yr's ago) my buddys father bought a 40 pickup with a 60 in it. It didn't take long for my buddy and me to ruin that engine. So we pulled the engine under an apple tree and install an 85HP that we picked up. I know we didn't change trans. so the 60 must have had the 1-3/8 spline clutch. Walt