You have to look up a local driveline specialty shop in your area. They are still around for truck applications.
The way that I've done it in the past is call the shop that you decide to use. My shop tells me to put the engine/transmission and axle in the car. Then take the front yoke of the driveshaft (the front part that slips into the transmission) and slide it into the tail of the trans as far as it will go. Then with the rear end jacked up and supported by the axle (not the frame) so the rear leaf springs have the weight of the car on them, then measure from the centerline of the front U-joint to the centerline of the rear U-joint. Then they subtract 1" (for installation clearance) and that is the length you need for your driveshaft.
I once just gave the dimension to my driveline shop and they made me a whole new driveshaft "from scratch" including the u-joints and front yoke and balanced it. You can also take your old driveshaft in and they can shorten it and balance it with new u-joints.
Not something to do at home in your garage especially for a beginner. You have to have your driveshaft balanced or you will get a vibration.