[xj] Updated GM Alternator Info

(Please update the FAQ with this info.)

I have the following updates to the info regarding installing a GM
alternator into an XJ6:

  1. Using a small-case GM alternator instead of a large-case picks up
    about an inch of bonnet clearance, which could be important in many
    installations. The smaller diameter of this unit makes it fit better all
    around. All the mounting points are the same, and of course you will
    still need a double pulley.

  2. The belt should be 31.5 inches long, not 32.5 inches.