Hi AyJay
I think you can buy the pedal box ready made for about £100
Check out 3GE
Yes you can make Sierra pedals work, coz thats what I did when I upgraded my Roadster. But it certainly ain't easy...Defo found it harder than making the book ones.
The book pedal box may look a bit daunting, but if you make one bit at a time You'll find it all comes together quite easy.
The box I made when I first built my car >>>>VVV
FYI... the clutch needs to be made from 5mm not 3mm as specified
You'll find 3mm is likely to bend under heavy use