What are you going to do for a door? Overhead door requires a header of 15 inches. Less if you go with low headroom hardware. Stay here and invest in some good photos. The information you are getting is going to save you money down the road.
I would think using some thing lighter than cinder blocks for a door would make it a lot easer to raise,cinder block garage door
Pictures would tell 1000 words. What about just raising the whole roof system? And about the door, you can always put a wooden header in if necessary, just have to take out a few more block.
Altering any ready-made truss is a BAD idea. If you need additional ceiling height, you should consider raising the whole roof system.