Here is my experience with the trunk lid supports on my 2600 Spider. The lid would stay in the half-open position with the original springs inside the struts. The springs are about .700 inch (17.8mm) outside diameter with wire size of about .112 inch (2.8mm) and about 20 inches (51cm) long. I never found a source for that exact size spring in the US, so I bought springs from McMaster-Carr in Atlanta, GA, USA. They are .750 inch (19mm) outside diameter, with wire size of .108 inch (2.7mm). The coils are slightly closer together than the originals. I think the minimum order was six pieces 36 inches long, so I had plenty to play with! The new springs were a snug fit in the strut tubes, but they would still work. By making them as long as possible, so as to avoid coil bind in the closed position, the lid would stay about ¾ open. This was still not good enough. My ultimate solution was to re-fit the original spring in one side and fit a modified gas strut inside the other. Unfortunately, the gas strut is so much stronger, that the trunk lid fit in the closed position was upset and had to be corrected. I think I put the gas strut in the side with the adjustable limit-stop. I had to adjust the limit slightly lower so the front edge of the trunk lid would not hit the hard-top. With just the soft-top, you can let it open a little more. If you resort to this fix, be sure that the strut attachment points are solid; no fatigued metal or cracked welds. You do not want one of these gas struts to break loose and hit you in the face. Now, I just unlatch the trunk and let go and it rises slowly to the limit-stop; it is wonderful, and no one will know it is not original. If someone would like to try the springs I bought from McMaster-Carr, I have two pieces 36 inches long. You can have them just for the cost of shipping. I am in North Florida, USA. Maybe your trunk lid has less lead filler than mine, and you might make them work.
Larry