went down this route recently, it has been resurfaced going over the hill making it an even better ride.
Start at Machynlleth Hospital. Take Forge road to Dylife. Stop at Wynford Vaghan Thomas memorial near top of mountain.(excellent veiws) Continue to Staylittle past Llyn Clewedog into Llanidloes. First part is a 'mountain road' often closed in winter and can get slippy in the wet. After the T junction just past Dylife the road improoves and is quick in places.