I believe as londonsean says that the mk1 rack has an imperial thread so would need different extensions (internal thread imperial/external metric to suit tre)
Most people buy new mk2 racks from rally design.
The book extensions are for a modified rack so are too long, but the alloy extensions from rally design are perfect for the job, as are the alloy rack clamps they sell (im not on commision honest!
hopoe this helps
ps. do you want rally designs phone no.