As I haven,t got the book with me,
The distance between U1 & U2 on the top face of BR7 if i remember correctly is 261mm ( i think thats correct, but check against the book diagram please),
Its a long time ago that I built my own chassis but don,t recall any problems fitting U1 & U2 , & yes my chassis is 305mm in overall height too...
I was never the best at maths
so you now have me wishing I had my book with me
cheers
andy