Best way is to make a hook out of a coat hanger and use it to pull the springs through from the back. Use your other hand to guide the spring if you can't see it.
Then you ca try this. If the headlight adjusters are functional, not rusted up or frozen, run the adjusters almost all the way out to releive tension on the spring. THis is how I nstall them. It sould get loose enough to slip the spring hook over the catch then tighten up both adjusters again, and re aim the headlights.
The way I do it on my 70 is I attach the spring first and then use my hand to pivot the lamp up and then do the screws, the one opposite to the spring first, but then again, mine are pretty rusted out.
Then you ca try this. If the headlight adjusters are functional, not rusted up or frozen, run the adjusters almost all the way out to releive tension on the spring. THis is how I nstall them. It sould get loose enough to slip the spring hook over the catch then tighten up both adjusters again, and re aim the headlights.