The alloy rack clamps from Rally Design come with packer plates that go under the pinion casting.
These ensure a rock solid fixing
A good quality product
My trusty Haynes manual ( 1982-192 ) lists the following data for the Sierra front disks.
1.3 & 1.6 fitted with 240mm solid front discs
1.8 & 2Litre fitted with 240mm vented front disks
As Andy said, any caliper type/part numbers may be usefull to ensure the correct discs are bought