Follow the Leangains plan as espoused by Martin Berkhan: 16 hour fasts, trained fasted, lift heavy weights. Eat low fat high carb on training days, low carb medium fat on rest days (with overall lower calories). Check out www.rippedbody.jp.
Why not gain muscle and fat at the same time? If you go for rapid muscle gain its hard to also avoid fat gain as you need to maintain higher levels of nourishment. But thats ok, once you are cleared and safe, go back on a more strict paleo that preserves lean mass but lets you begin losing the fat. Voila, awesome body!
mate when i was gaining weight i just used to do those weight gain shakes, do the gym 5 days a week and eat like a horse. if you have access to anabolic steroids or GH this is the fastest way to put on weight, probably not the safest. Train a lot eat a lot and rest. Concentrate on compound excersises: Bench, squat, chins, dips etc