Do you have to ground your 351 ford motor for it turn over?

No, you don't need a dedicated ground wire *to the engine* for a 351 Ford (or most other engines) to turn over. The starter motor's ground is typically achieved through the engine block itself. The block is bolted to the engine mounts, which are bolted to the chassis, providing a ground path.

However, a good, clean, and low-resistance ground path *from the chassis to the battery negative* is absolutely essential for the entire electrical system, including starter operation. A poor ground connection will manifest in various ways, including a slow-cranking starter or the starter not engaging at all. So while you don't ground the engine to make it turn over, you *do* need a good overall chassis ground.