Welcome cjef,
I had some front and rear coil springs specially made for my '65 Skylark GS. I wanted it to set up 2" higher all around from stock height. For your application, you could go with a stock front spring for the '65 Skylark GS. I'm assuming that the 401 nailhead would be close in weight to the 455. However, you may want to call these folks and explain what you have and want to do. They are very knowledgeable and helpful to help you get exactly what you want. All their springs are specially made. Price is comparable to standard aftermarket springs and they have a reasonable shipping schedule.
Coil Spring Specialties
632 West Bertrand
St. Marys, KS 66536
785-437-2025
Remember that it's more important to get a stiffer spring, not a longer spring. They need to fit in between the A arms. While you are at it, it would be a good time to replace those worn out A arm bushings. Good luck.
John Egel