ORIGINAL: mjr46

84's came with exhaust manifolds I believe and as addermk2 stated had single exhaust and a crossmember will be needed from a 86 and up stang and the 87 headers may not be a direct bolt in without exhaust modsbecause there was on the right sidemanifold I believe a vaccum controlled exhaust flap built into the manifold set up which changes pipe dimensions/ line up a little.. this flap was to aid in faster warm ups........but also with all the kinks and bends in the stock 86 and up headers the ports are very small and restrictive and may not give you much of a performance difference but more so better look