How i can build a headache rack for a Chevrolet 1891 (3/4 fullsize
Pickup) myself? (Welding, Metal, Mesurments)

I would try fiberglass as it is a farily easy material to mold. You
can purchase fiberglass at just about any auto store.

Personaly, I would use metal. I made one about 15 yrs. ago out of 1
1/2 inch square tubing and something called "expanded metal" this is a
type of metal that you normaly see on homemade bbq pits. Create a
frame that is about the shape of the rear window. Make it bigger
ofcourse, have it follow the sides of the cab "looking from the rear
of the truck". You can make this frame from 1 1/2 inch or 1 inch
square tubing depending on your taste. Then cut the expanded metal to
fit the frame and weld it on. Just look at one on a truck somewhere
and you will see how it works. This is a good project for a weekend
welder.

Here is a photo of one so you can get ideas. 

http://www.ustruckaccessorieswhse.com/go-toolhead.jpg

Good luck. Tim