If you can find a 1993 Caprice 9C1 cam, that should give you about 20-25 HP, There are a lot of other cam options. You should change heads, and intake manifold as well. I do not know the laws for cars in France, so I do not know what you are allowed to change without getting in trouble.
How is putting a 350 in the car illegal in France?
I believe the 84 has a carburetor, but it may be computer controlled. Getting more air in and out is the idea.
Long tube headers, assorted cam shafts, heads, intake manifolds, larger carburetors, bore and stroke increases, electric fans...they all add up. If it has a carburetor you can modify a lot more without getting into computer BS.