Ok, I had a shady contractor start an attic build out for me. my attic had 2x4 trusses on 24" center. The contractor put in a 2x6's nailed to the 2x4 in the roof, top brace. Additionaly a verticle knee wall support is resting on a 2 - 2x10 beam spanning between 2 load bearing walls. The flooring is 2x8's on 24" centers and 3/4" tongue and grove plywood. If you look at the pictues you will see the first 10 feet have been done and the remaining trusses need to be reinforce and cutout.
I am doing something similar, but I do not have trusses, just 2x4 rafters 16OC. An architect friend told me to go with 2x8's sistered to the 2x4 rafters and to nail 2x4 gussets like yours, but nail them on the 2x8s. I have not had a chance to ask him, but I would think the sistered 2x8s should go from the ridge board/peak down where it can sit on the wall/floor, not just part of the way like in your pics. One ? I do have is this is true about bringing the new rafters down as far possible, does one cut it to fit into the opening just under the roof where the old 2x4 sits on the wall or just as close as possible.
Sorry to hijack the thread