Has anyone got the plans for making brackets for fitting sportpack seats into the wider, earlier seat mount. I've seen one on a forum but can't find it now

 

The inner holes are used without modification.
The outers need to have extensions welded to them so the holes are moved the correct amount inwards to fit the seat rail. This is easier to do accurately if you remove the squab from the frame and set it up with the inner runner of the rail bolted in place. All carpets out, obviously.
It is a surprisingly tricky job to do properly and safely. I seem to recall seeing a thread about it that featured what I felt were appallingly gimcrack bits of steel that I felt would have made it far from certain that the seat achorages would hold in a collision. The fixings are safety critical.