Not as much as you would think, the cars in those days still used a stock frame but with renforments and the roll cage tied into the frame.
Not as much as building a whole new car which you would have to do for any newer era stock cars. Its just taking the existing roll cage and adding a front and rear to it.
I love the group b cars now in beamNG , but i tink the Lancia Delta Integrale and Peugeot 205 T16 are cars that need to be in the mod too
Sadly they don't just "make" cars. It takes months, sometimes even up to 4 years for a quality car to be crafted. So don't hold your breath. Remember this is a feedback thread, not a vehicle suggestions thread.