Hi Mark,
170A cable should be OK, noting that the CB750 starter typically draws 120A ... but 280A at stall.
Take care if simply earthing to a frame point... as all of those above 120-280 amps will have to pass through this connection on their way into the frame... and thence through the various engine mount bolts & to your starter. (which is using the engine casing as an earth)
For my money, you'd be better, if possible, doing it the Honda way & choosing a nice big bolt which goes through a very solid frame point & also (electrically) bonds the engine casing. Wherever you decide to attach this, -bearing in mind the loads it carries.. scrape away all paint etc & get the connections down to bare metal. Don't pick any bolt which goes through vibration mounts - as these will insulate any current from flowing.
If a good connection is not made you run the very real risk of melting your loom (earth) down the track... see the photos as posted on this forum about a year ago.
Cheers.