The last meet we had down here in the summer, we did the A4069 from Upper Brynamman to Llangadog then onto Llandovery and Builth Wells, dropping back down to Brecon and then heading south on the A470 to finish.
They're all pretty good roads around the Beacons so take your pick.
Keep us updated on your intended route and timetable as a few of us might tag along