from a n/a engine to a turbo engine, you will have to take it to a tuner to be setup as the ingition curve is different, dont know much about the 1800 lump but i would say no more than 6psi.

Nowadays there are alot of companies who sell thicker headgaskets designed for your conversion to lower the compression of the engine to allow a bit more boost, speak to Van alken they are good at turbo conversions on zetecs etc