What is involved in replacing the standard rst t3 with either a cosworth t3 or t34?

Cheers

 

Cosworth fitment turbo's will need a different downpipe as the bolt pattern is different and they don't use the rst elbow. T34 may need a spacer on the manifold too to stop it fouling the manifold primaries.

 

to fit a cossie t3, is just a case of changing manifold from cossie to rst, simple job.
and of course the down pipe needs changing as it's a flanged one onto the turbo not to the rst elbow which is restrictive.
the rst elbow can be used, but i wouldn't recommend it.
as for t34, direct fit yet again IIRC as the cossie t3 is.