that doesn't really look like it's going to clear 650bx50, maybe 40mm? If you can get a 40mm tire in there, it isn't a bad idea. It really depends on your road surfaces. Around here, there seems to be a pretty big budget for gravel, and the roadbed base is 3" rocks, so bigger is definitely better. With 30mm tires I was getting regular pinch flats. None this year on 38mm tires. My guess is you can go with 40mm slicks and not suffer much.