home build exhaist system advice needed

finally my car runs good so back to the fun part :D
im starting to build a stainless steel system i plant to make the manifold from 1.5'' and into to 2.5'' but i cant seem to figure out if i shall make the manifold short and put the cat against the engine as the original or make it long and put cat under the car what do you think makes the best performance
and i i have been seaching for 1 last part witch i cant find a stainless steel connector witch goes from 4x 1.5'' into 1 2.5''
i after the cat there will come a middle muffler and at the end 2 one in each side
any ideas to get best performance besides my plan :D