Your T-18 struts probably had a diameter of 2" or 51mm. So you need to buy a coilover sleave that is very close to that size. And you probably want the adjustable spring seats to suit 65mm ID springs. The coilover conversion you are doing is a conversion, something to be thought about, not just something for every kid to bolt together. Often when building a coilover strut, even if you have a kit, you will also need a lathe, a welder, and a brain.