Heck, I've pulled trannies with 11" clutches out of early '60s trucks with Polys, so sometime before then. My '66 Sweptline 318 Poly has an 11 inch clutch, as did my '78 D200. The '89 NV4500 I've got uses an 11" (going behind a Gen3 using an eight-bolt 11" 4.7 flywheel).