Hi,
Start with the outer beams and work your way in inwards for the lengths you're missing. Look at a good strong chassis like the Southern GT and make your ideas. the bodywork gives it self as to widths and heights, if you have a body to work from, and build your chassis as you go along. triangulation is easy and you don't need to calculate everything in CAD, its enough to work with Cardboard Assisted Drawings, look at other chassies for inspiration.
Or just buy a ready made Southern GT chassis and be on your way.
I started like you and luckily binned my first GT40 project after wasting years on it.
Morten