I've done a few of these a fewyears ago on mk2 rs 2000's and a 3dr look a like ...followed the same method as that s2 in the link...

we used good second roofs cut off at a scrapyard and spent most of one day cutting and drilling the weld and pillars off to be left with just a roof skin before cutting off the roof skins off the mk2's ...we done this on a mk2 that had a pop up sunroof in that we wanted rid of ...the 3dr one was a clart cos we had to run the drain chanels down inside the pillars ...

gently run the grinder down the front pillars (as photos on the link) till you see the brazing (that's your cut mark)then drill out the spot weld along the sides(gutters) and inside the front and rear screen or tailgate area....

to be honest it took longer to seperate the skin from the old pillars on the donor roof and prep the skin for fitting than what it did to put the new roof on ...