I would take a cab. If you figure on about an hour for dinner, at least, then you won't be done until 8:15 or so. Then you would have to do one of several things...1. walk to MGM, via the walkway, taking about 15-20 minutes. Then wait for the bus, could be up to 20 minutes, then the drive to POR, about 15 minutes. Cutting it close.
2. Take the boat to MGM...but unless you get right on one that is just leaving, it could take you close to an hour to do that. It's generally a 25 minute trip once you get on the boat to MGM.
3. Take a bus to a park, MK for example, then transfer to the POR bus...taking about an hour also.
4. Take the cab...about 20 minutes. Gives you plenty of time, stressfree. Bell Services at the BW should be able to do that easily for you.