I suppose a threshold question that you should ask is why you want to change to a five speed. As Scott commented recently, the first four gears in the 265/6 are the same as the gears in the four speed. Thus, the primary benefit of having that 5th gear is for long drives at highway speeds. Consider how you are planning to use the car and where you live. If the vast majority of your usage is local trips and shows, the trouble and expense of the conversion may not be worth it.
If you put 4000 miles per year on your car and you get 20 mpg, you are using 200 gallons of fuel a year. Assuming $4 a gallon, you are then spending $800 on gasoline.
Now, if the 5 speed improves your mpg to 25 mpg overall (which would be a lot), you will save $160 in gasoline annually. Compare that to the cost of the 5 speed swap.
Plus, the getrag 265/6 is becoming more and more difficult to find, and it is very difficult to rebuild them because parts are very hard to find.
If I had a four speed in my car, I'm not sure I would change it. And if I did, I would make sure to keep the four speed on hand just in case.