I'd stick to something that is in keeping with the period look of the car.
How about a set of Mk 2 Jag wires? 5" X 15" gives you a couple of tyre options but doesn't mean you need to be Arnold Schwartzenegger to park up. OR get your hands on some minilites 5.5" X 15".

Try: http://www.mwsint.com/ who will sell you spline adaptors for the P1800S

I have them o my '666 and they look the business!

Or try: http://www.minilite.co.uk/

These wrere the wheels on 'The Saint' car

If you want to see some wires on a P1800, check out my car at:

http://volvo1800pictures.com/DOY/gordon_main_frame.htm

All the best,

Gordon