Hi Bob
You can get the rack extensions from a number of places
Rally Design do them , Saturn do them, I think Tallon has them at times & Dave oxley on ebay quite often has them listed.
Some are alloy & others are mild steel so that's the first choice you have to make
You'll have to jog my memory on donor cars, Sierra or MX5 but be aware I am almost certain sure that the book length extensions are too long
I have a feeling that the book car used a shortened Escort rack & needed longer extensions ( Sure someone will confirm that for definate )
On my own Sierra based build I had to shorten the book spec extensions.
Rally Design & Saturn do the correct length alloy ones, I expect if Phil is doing a batch of mild steel ones, he will make sure that they are the correct length.
Hope that is of some help