I played LB also, but in college. I added 20 pounds in one summer up to 245 and ran a 4.45 40 (down from 4.6) before I was injured and could no longer play (rehabbed, but kept re-injuring). Here is what worked for me (in the weight room stuff, not including running and agility, etc.):
- increased speed - speed squats, which means take about 50-70% of your max and squat as fast as you can in a controlled way. Make sure you stay in control. 10-20 reps for 3 sets. This should be on your light leg day, and it also will really get your heart pumping! No cardio needed after this workout!
- increased strength & stamina - supersetting - use heavy weights (90% of max) for as many reps as you can. When you are exhaused from that, use 50% of the previous weight and do as many rep as possible. Take short breaks between sets (1 to 1 1/2 minutes). Also, try to go up fast, and down slow. i.e. bench - up as fast as possible, down in 5-10 seconds; pulldowns - pull down as fast as possible, up slow etc.
Hope this helps and good luck.