I didn't start this discussion to stir things up but I am happy the way the discussion has developed. I also posted in the HSC thread rather than HMC because it is specific to FIA HTP or Sched K regulations.

My understanding of those regulations is exactly as Steve suggests, if you can categorically prove it was done in period (specifically in an internationally recognised race or series) then it is allowed. On that basis, the original tubbing of the Broadspeed cars is allowed because the current owner and/ or restorer can and will prove it. Which is not to say that the proof is widely available to other car builders or restorers!

On the other hand, the horsepower that some "historic" cars now have is a huge issue in historic racing in the UK and certainly has the potential to occur here. The suggestion of declaring the power outputs and benchmarking them to what was achieved in period has considerable merit.

Possibly the requirement of running the correct gearbox has the unintended benefit of keeping horsepower in check but not always.