I have another thread about using spacers when switching from the stock XJS wheel to and early XK8 wheel. I found this handy device to check the differences and fit. According to this, may not need a spacer at all.
Stock wheel: Diameter: 15", width: 6.5", ET: 28.5mm
XK8 wheel: Diameter: 17", width: 8", ET: 33mm

Rim & Tire Size Calculator. Custom Offsets - Wheel-Size.com

 

The xk8 wheels will run on the front when turning at max lock without the spacer. If you get a spacer make sure you use a hubcentric spacer.