Not sure I agree with RangerSVT on this, running two wires would be OK but I don't think it would be "better".
Current stock setups run 90-200amp alternators via engine ground, not separate ground wire to alternator, same with the starter motor, it is not a separate ground wire hooked up inside the starter, just a case ground, but cable size/gauge will matter.
The frame is only a "good" ground if the battery "-" is connected directly to the frame with a larger gauge wire.
Usually the battery "-" goes to the engine for the starter motor, highest draw, and engine is isolated from the frame.
It will have smaller ground straps/wires to the rad support and engine to firewall but nothing very large.
So get another short large gauge cable to run from that engine ground to the frame, nice clean point on the frame, and engine moves a bit so not a tight cable hook up
And the Frame is only a good ground if it is metal that you are connecting to, not paint, undercoating or...rust, so clean it, bolt it on, then paint it to prevent rusting.