I've done this in other vehicles a few times. It can be done right and be just as good as factory if you know what your doing.
The advice about finding a complete donor car is some good advice.
The most important thing that you will run into is the hole in your floor board. It seems simple, but this is the area you will run into trouble when you think you've thought of everything!
The best trick I can offer you is this. Take a big enough sheet of paper (or 3 in case you mess up a few times), and a brick of charcoal. Find a factory manual XJ, pull back all the carpet and insulation to get to the sheetmetal, and make a template of the floorboard. Just like we used to do with leaves and stuff in kindergarten. Your brake rod will be in the same place. So will all the contours etc. Then you will know exactly where and how big to cut your clutch rod hole in the floor. You will run into a poor feeling clutch, fast wearing bushings, or (at worst) broken rods if you don't.
Ask me how I know. :gee:
Hope that helps.