Not sure if this will add much to what has already been covered but my Dad ran a 1200 rally car with a Reg Cook 1298cc A12 and a close ratio 56a 4 speed (not ultra close). There was one or two gears that were straight cut. At the time-mid 80s when he got it off Cook no one knew who did the gears for him apart from the fact that the mans nickname was Herman the German.

He told me that it was a 120y box and it was stronger that the 1200 4 speed - I didn't know there was a difference between the two 56a boxes but back then most things were word of mouth. It never let him down but he always said it was the weak point of the drive train.

Dave, I am quite surprised at how reasonably priced this kit is and look forward to hearing how it goes!