Cut the bottom (inside) of the cowl out of it. When I had the one from my parted and dead 76 stang II out, it must've weighed 50 or more lbs! There is a LOT of weight in those doors too, I am assuming you are trying to keep the doors functional, but it may be better just to weld the skins onto the outside of the rest of the body. If you want to lighten the whole thing a bit, try swapping the full rear diff for a straight axle out of a FWD minivan. Not sure what would fit, but could be fun to peruse the pick-a-part yards with a tape measure. It couldn't be too hard to weld spring perches on one of them.
Good luck, and have fun!