Hi Dave
HSR has their own interpretation of what is Historic and they clearly have a sweet spot for P cars. HSR, even though they will tell you they are strict in terms of car originality, is well known in North America by those who overbuilt a car that doesn't comply anymore to the usual Vintage/Historic norms. HSR sees no problem usually to accept an early 70's (2.2 litre engine) P car with Turbo 3.6. If one's car is not period correct anymore, then go to HSR. And usually it goes to an extreme. It is fine too, if it fits what you like.
I am not a sucker for originality, but let's say that HSR's interpretation goes beyond what I can personally accept. If you are into great Vintage/Historic racing, then look for Pittsburg's Vintage Festival in the summer.
http://www.pvgp.org