it is trusses, so you MUST get an engineer to look it over first. what seems minor to a casual observer could have catastropic effects.

are the floor joists even rated for the new load of a room above?

If the floor framing of the new attic is strong enough for a bedroom load, your best bet is to duplicate the framing of the new roof and attic floor next to the existing trusses and then cut out the parts of the trusses that are in the way. It would be wise to consult with a design professional so the existing conditions could be investigated in order to avoid any structural blunders.

Forget the strongback and 2x6's unless you know how to design a truss including the connections. I suspect that the house would have to be very narrow for 2x8 floor joists to do the job. How did you arrive at that size?

the new addition ceiling joists are 2x8's and the trusses in the old section are made out of 2x4's. the longest span without a wall interaction is 10'. the ceiling joists will sit on atleast 2 walls for support. and the 2x8's will be nailed to the ceiling joists section of the 2x4 trusses.thus making the 2x8's the new ceiling joist where the trusses were. they are all 24" centers. the attic addition will be almost equal in area from new section to old section (attic space).

I would get an engineer to look at it too. Better safe than sorry!!

The big question is whether or not you plan to remove any intermediate truss members.

I don't know how they are positioned but, in general, they are designed to be as efficient as possible and therefore the removal of intermediate members could lead to collapse of the roof if enough trusses are modified.

This is definitely not a DIY project.

Is there a window for egress?

Veteran firefighters had to jump to their death because a fire they were fighting in a Bronx building had no egress - Reason? greedy landlord blocked the windows with partitions in order to increase his revenue by illegally adding more apartments
Perished decorated veteran fireman - with 7 children between the 2 - Imagine the horrific way these 2 perished- deciding to jump 50' to concrete or be engulfed in flames

http://cms.firehouse.com/web/online/In-The-Line-Of-Duty/Firefighters-Killed-in-Bronx-Blaze-Were-Forced-to-Give-Up-Their-Hose/39$38599

Egress is literally defined as "a path out" or "to emerge," and these terms directly apply to the design principles behind egress windows. In case of an emergency, windows installed in any sleeping room must pass strict codes so that occupants can exit and rescue specialists can enter the bedroom. In other words, when a fire occurs, your bedroom windows have to be of a certain size so you can escape and firemen can come in. These requirements also include basement egress windows if you have a finished bedroom below.

The Regulations
Remember, when you're adding a bedroom to your home or basement, you have to keep certain requirements in mind:

Minimum width of opening: 20 in.

Minimum height of opening: 24 in.

Minimum net clear opening: 5.7 sq. ft. (5.0 sq. ft. for ground floor)

Maximum sill height above floor: 44 in.

Did I miss the OP saying he wasn't going to have windows?

well since he didn't come back following the window ? and did not state in any postings about a window - I'll take a guess - No