do you mean you want to fit a t3 from a cosworth to an rs turbo?
if so then you have a few routes to go down.
you can just unboltthe core and attatch it to the escort rs turbo turbo exhaust housing, this will boost more, but will still have the limitation of the 0.36 housing, but you can get a 0.48 housing and retain near the cossie housing potential.
alternatively you can do as pictured above with the complete cosworth turbo unit fitted to the escort rs manifold, just a modified cossie downpipe would be needed.
i did the latter for mine as it flows better than the rs turbo elbow off the turbo unit.