Kbev, doubt you'll never improvement in your climbing speeds by lifting heavy weights slowly for a few reps; that work builds the wrong kind of muscle. For sustained hill-climbing, where you have to push the pedals forminutes at a time (hundreds of reps), what you need to build up is your slow-twitch aerobic muscles, as well as their mitochrondria which converts glycogen into the energy that enables them to repeatedly contract. You also need to build the capillary and venous network to move the O2 rich blood in and clear the lactate out. Fortunately, riding training 300 km's per week will definately do all this.
Heavy lifting in the gym may be fun for a lot of us (I like it), but sadly it has really nothing to do with bike riding, where you need to push no more than your bodyweight for hundreds/thousands of reps non-stop in one "set". Ask yourself this question: does a powerlifter train on a bike to improve his VO2 max (aerobic fitness) levels as a way to lift heavier weights?