Hi
You can fit 205/70VR15 and there are good tyres like Pirelli Cinturato or Michelin XWX. However these earlier cars handle better on 185VR15. And I would suggest Pirelli Cinturato. After all that is what Jaguar fitted if you asked for radial tyres on your series 1.
Incidentally when the series 2 came out, Jaguar were fitting 205/70Vr15 tires to the XJ6 but they didn't fit it to the XKE because it didn't handle as good.
Www.cinturato.net