I assume this is just a street car.
If the back is at 25" he is ok there. If there were not handling issues or premature wheel lock up the back alignment may not need to be touched.
Lower the front to near 25.5" as Chuck noted with a half tank of gas and your front air pressure set correctly. You should be ok but may not have enough adjustment and may have to re-index the front bars to do so. After lowering, drive around a bit to settle the car.
Once you have the height where you want it do some fast stops with the tires warmed up. If one wheel wants to lock up easily, your corner balance may be way off. If so, do a search for "tripod method". I think Chuck came up with this. It is basically jacking the car up by the factory jack point, taking the rims off that side, lowering the car back down to stock height on the driver's side. Then you measure the distance from the ground to the fenders or better yet the center of the torsion bars and then compair them to the other side. This should get you close for a street car. If the rear distances are not close, you might just want to take it to a shop and have them set up the car but you can do this your self.
Aligning the front, if the back is ok, is not to big a deal. You can reset the toe using a tape mesure. If you do not go below 25.5" with normal xize front tires you will pick up some neg camber in the front tires. That may not a bad thing and will make the front hold better in corners. The front is easy for most shops to align. Call the local Porsche repair shops to see if they can recommend a shop that knows these cars.
Most of these older 911's have issue with bushing sag that should be looked at. This sag can dammage your torsion bars. If one needs new bushings or is adding new shocks that rase up the car, it is best to go all the way and have the car professionaly set for height, aligned, and corner ballanced.
Just my $.02.
