What always worked for me was to do sprints/condirioning in the AM and weightroom work in the PM. It can be tough to sprint after squatting, and the time in beteen to refuel before lifting should leave you plenty recovered. I can't sprint on off-days because it leaves my hamstrings too blown to get an effective workout the next day.
Make sure to learn and practice the olympic lifts, they keep you explosive, quick, and flexible as you pile on bodyweight, it's the key to cat-like quickness and heavy bodyweights in my opinion.
I'd also recommend plyometrics, and the occasional pick up game of basketball, or tennis or raquetball or something like that. Things that require athleticism, foot-speed, and grace and coordination help you stay quick and athletic when packing on muscle too. Maybe sprint twice a week and play pickup b-Ball or tennis one day.