I am not completely discounting methods outside of weight lifting to get stronger on the bike. In Greg LeMonds book he details how he trained by doing slow rolling accelerations in a 53 x 13, for 30 seconds on Tuesdays. There are also single leg isolations where on an uphill, under 5mph, unclip one foot and leg press with the other one for 10 reps, and switch off. Do not spin.