Generally, you want it as short as possible. So a convenient and close spot on the frame rail would be best rather than running it to some remote location. An engine mount bolt (frame side) comes to mind. It'd be easy enough to drill and tap a hole for a large bolt or weld a bolt to the frame if that's an option. Use plenty of dielectric grease on whichever you do.
But, is that big ass bracket that the alt is bolted to not enough of a ground? Assuming it's a stock alternator, the body is it's ground, and it's bolted to that bracket, which is bolted to the engine- all rather large chunks of metal. The engine could use a better ground to the frame, which is part of the "Big 3" upgrade. It'll be much shorter than running one all the way from the alt.