You didn't say which engine you're talking about. %)
But if you mean a smallblock V8, there are actually 2 ground bosses on the block:
Always clean EVERY electrical connection surface, and apply a thin film of electrical grease to keep it clean longer.
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It's not necessary to weld anything to the frame to ground it properly - there are already some single-walled gussets behind the engine crossmember if that's how you want to attach your ground cable. But on Frank's '75, I just used a later green self-threading ground stud in the outer surface of the frame rail near the front of the passenger wheelwell so it's easy to access/check/clean, and close to the front if a winch was ever added. Some eB frames already have holes there.