ive welded a few of these in, in the past and here is my advice remove the old one and fettle the new one in from there, leave a tag of chassis at etheir end to weld to and as a guide, as the old one still rots away underneath and in three years or so you will be doing it again, irrespective of how much wax oil you put on as they rot from inside out. its up to you, my mothers mk3 cortina had three sets on before i realised and the last set have been on ten years an are still ok, the job was alot worse the 4th time as the floor was getting fooked up with all the welding etc from the past attemps.... your choice