That is going to be extremely tight. I think the concerts are scheduled to last 35 minutes. So that puts the end at 6:05, and you trying to get out with the crowd. Those concerts are held on the stage across from the American Pavillion, so you are right in the middle of WS, which means any way you try it- it is going to take a while to get transportation. You really have 2 options:
1. Walk to the International Gateway and out to Beach Club and get a cab--HOWEVER, realize that it is a 5 mile drive which typically takes about 15 minutes and that does not count the time it is going to take you to walk to the Beach Club, walk through the building, and get a cab (there might be a line for cabs with people going to dinner at resorts about that time- or there may not be a cab waiting and they might have to call for one). Realize that if you started at one point and made a circle and got back to where you started in WS, it would be approximately a mile. I could be wrong, but I think the walk from American Pavillion to the door of the Beach Club lobby is approximately .6 of a mile (not counting walking through and getting to the cabs). We've stayed there and made that walk more than once.
2. Walk to the front of the EPCOT, catch the monorail to TTC and walk to the Poly from the TTC. I think your walk to the Monorail station is going to be about .7 of a mile- close to the same distance as taking the turn to walk down to the International Gateway and out through the Beach Club lobby. Assuming there are no technical issues, your Monorails are consistently running, and hopefully your wait for the next one is not too long. That time of day, there typically isn't much of a line to leave EPCOT by monorail, so you should be able to grab the first one that arrives. The monorail bypasses all traffic, and it is about an 8 minute ride (as opposed to the 15 minute cab ride in traffic). The TTC is right next to the Poly and has a cut through to the Poly interior paths (which I assume survived construction) and the walk to the Poly is just a few minutes. Before all the changes we used to stay at the building at the Poly closest to the TTC and it was about 2 minutes from the time we got off the monorail until we got to our room (fantastic after a long EPCOT day)- the walk to the main building is farther but still just a few minutes away.
I'm not sure time wise, either option is necessarily better. It would depend on the availability of a cab and traffic- or whether you just missed a monorail leaving and have to wait for the next one or if the Monorail line is having any issues. Either option, it will be tight to make the ADR If you are lucky you might make it to your ADR in time, but there is also a good chance you are going to be late.