Mass building exercises:
Legs: Squats, Dead lifts, Quad Extensions, Hamstring curls, Leg Presses, Standing Calf Raises and Seated Calf Raises
Chest: Barbell Presses (Flat, Incline and Decline), Dumbbell Presses (Same)
Back: Seated Rows, Barbell Rows, Dumbbell Rows, Lat Pulldowns, Still Leg Dead Lifts
Biceps: Barbell Curls and Dumbbell curls (All variations)
Triceps: Nosebreakers/Skullcrushers, Pressdowns, Dumbbell Kickbacks, Weighted Dips
Shoulders: Barbell Presses (Seated), Dumbbell Presses, Front Raises, Side Laterals, Bent Over Laterals
Forearms: Wrist rolls (variations), Reverse Curls
It's not to say there isn't room in your routine for other exercises as well but I consider these the basics. Do them with correct form and as heavy as you can and if you eat right and recuperate properly....you'll grow.
At 6'4" 260 seems like you are pretty muscular already. I can't really tell. You might have more body fat than you'd like but still there has got to be some muscle there judging by your photo even though you are fully clothed. You don't look out of shape at all, that's for sure.
The diet, strict in the sense you should be eating clean most of the time. If you're bulking and you eat tons of pizza and burgers and fried chicken....yeah it's protein but you'll get fat and that's not what your looking for. Baked or boiled chicken, fish, lean meat, turkey alll skinless and trimmed of as much fat as you can possibly eliminate from it. Don't cook in vegetable oils, use olive oil (small amounts go a long way), safflower oil, flaxseed oil. You need fats to build muscle but you get enough from food just as they are you don't need to add more in preparing.
When I was in training and bulking I was eating mostly egg whites, chicken, tuna, turkey, salmon (one or two meals a week) and lean red meat (again one or two meals a week), those were my proteins. Carbs, veggies pretty much all you want, sweet potatoes, oatmeal, brown rice and a bit of fruit for breakfast and post workout. Used some protein powder pre bedtime along with a "snack" of cottage cheese and a few blueberries or strawberries. That's about it. Six to eight meals a day. I was up to 5,000 calories a day for a while and went from 167 to 194.5 at my heaviest before cutting down for show to 174. You'd definitely need more because you're a foot taller, almost 30 years younger and already outweigh my start weight by about 100lbs!