There are a variety of tools for compressing a coil spring to install it between two A arms. Relatively inexpensive tools with a threaded rod having movable hooks on each end are commonly seen. Some are designed to go inside the coil spring and some are designed to hook on the outside. If you can get the tool in a convenient position it may work.
There are more expensive hydraulic and pneumatic tools that make it easier to compress the coil spring. These are used mostly by professionals who do a lot of undercar work. The cost of these tools may be excessive for the amateur who does this job very rarely.
A technique that we have used in our shop with repeated success requires a strong, ratcheting style tow strap and a good floor jack. You can hook the tow strap onto the jack or you can slide it under the jack and wrap it completely around the jack and the suspension.
Seat the spring in its upper holder, then bring the A arm up to the bottom of the spring so that the lower spring seat will catch the bottom of the spring and hold it in position as the seat on the lower A arm is pushed upwards. Then, raise the jack against the bottom of the A arm, making sure it is in a spot where it pushes squarely against the A arm. Sometimes a block of wood helps position the jack just right and protects the paint on the A arm.
The jack will not raise the frame of the vehicle because the straps are holding the frame to the jack. And the jack will not lift, because the arm with the jack pad on it is pushing against the frame and forcing the jack downwards.
Once you get the lower A frame raised to the right height, you can insert the long bolt that goes through the holes on the outer ends of the lower A frame. You may have to use a long screwdriver or a drift to line up the holes and tap the bolt slightly. Then, fasten everything up with all bushings and washers in place. Tighten the castle nut per shop manual instructions, slip a cotter pin through the “turrets” and remove the strap and jack.
We can’t guarantee this works on all vehicles, but it has worked well for us at least a half dozen times on a variety of cars. As with any restoration job, go carefully, think things out, do a “dry run” to test the equipment and wear proper safety gear like gloves and a face shield. Also be safe and cautious at all times. It’s no fun having an energized coil spring flying around a shop!