Not sure how the earlier cars are arranged, but if like my '94 4ltr. changing headlamp bulbs is a real pain involving putting the steering on full lock and removing a plastic panel under the wheel arch behind the headlamp, then inserting your fingertips and feeling around for the spring retaining the bulb and releasing it. Putting a new one in is even more fun as you have to heed dire warnings about not touching the glass of the bulb with your fingers. Best of luck - yours may not be like that.
Before you do anything have a look at
http://www.powerbulbs.com., an online supplier near you which I can thoroughly recommend, and get yourself some proper bulbs. I have been using Phillips X-treme for some time and they really transform the lighting of the XJS with rectangular lenses, making night driving a pleasure and fitting the 4-light conversion quite unnecessary. I see that a new generation of Phillips has just come out claiming plus 100% light output , and it will be a true claim. They sometimes have special offers on, so have a look and see where you are going .