Welcome to the forum nsbrit.
AFAIK the Rover P6 3500S was raced in the UK in period as a modified saloon. It had all of its bolt on panels made from fibreglass. Can't remember the name of the chap that ran it but I know a man that does. I will "pm" you with his name and number.
I believe Andrew Kitson may also recall the car and I'm sure a few of the guys on this forum will be able to help. (He may even have a pic or two).
As to your current needs, it would depend on what regulations you are running to. Over here there is a chap who races with the CTCRC and Top Hat 70's saloons who runs a P6 3500 and that is built to FIA Group 1 regulations. That means no body mods, standard carburation (in terms of numbers not size) and full race engines etc. You can get those regs from the CTCRC site at
www.csccgb.co.uk. Look for the pre 74 Post Historic regs.
I think the car may have problems with its inboard rear brakes but it seems to go quite well.