You are going to have to remove the valve springs to replace the seals if that is what is needed. When doing that bring that cylinder up to TDC so the valve don't fall into the cyl. if it's down. There will be two cylinders at TDC at the same time.
The engine rotates in a clockwise direction as seen from the front of the truck, and for every 1/4 turn of the engine(crank) each next piston will arrive at TDC. So, When the line on the vibration damper is at the 2 oclock position, cylinders 2 and 5 will be at TDC. When the mark is at the 5 oclock position, cylinders 7 and 6 will be at TDC, 8 oclock has cylinders 3 and 8 at TDC, and back to 11oclock, 1 and 4 will be at TDC.
Cylinder numbers are assigned 1,3,5,7 on the left (passenger USA) bank from front of truck to back, and 2,4,6,8 on the driver's side, front to back. Firing order is 1,2,7,3,4,5,6,8.