Ports are usually higher alcohol anyway, so the wine will be just fine even if it dropped more sg than you expected.
Best to let the yeast do their thing till they quit, and not try to stop them -- pretty much you can't stop them anyway, sorbate will inhibit their reproduction but not kill what's already alive and the sulfite isn't going to stop an active fermentation reliably either.
I second what Vcasey said - wait until the sg has stopped changing for 2 or 3 days in a row, then add the fpack and finings.
You will have a fine wine when this is all done.