I did this a short while ago. I used the mk3 rails and drilled and cut off the little bolt through tags on both rails. I then welded the tags from the mk2 rails onto the mk3 rails and slotted the seats right in. Just had to measure the distances between the holes. I have heard of someone doin this by just bolting the tags on but as i had access to a welder and someone that can use it i went for the extra strength.
Didn't notice much difference. Only that it adjusts much higher than the original seats. I found i needed to wind it down to pretty much its lowest setting to get a comfortable driving position and i'm only about 5'9.
made a start last night, what a effort. decided to strip them down to clean and treat the leather at the same time, the mk3 rails are considerably bigger. im going to weld them as well as if you bolt them in, it looks like it will restrict the seats movement.