"I ordered the standard 36 pound spring and a 38 pound extra power one. I would hesitate to use the 38 pounder out of respect for the age of the gun."
I believe you have it backwards, a stronger spring will save the old pistol from "battering" better than the weak one. A lighter spring would only be correct with lower power loads, JMHO.
I'd go for the 40 lb!
But just an observation- when I ordered a set of springs from wolf, 36-38-40,
I fully expected them to be different numbers of coils and/or different wire guage and or different lengths. When they arrived, they were all the same wire guage, two were exactly the same length, and one was a little longer, but all had approx. 20 coils. The 36 and 40 were the same and the 38 longer. I have no way to measure the tension, but it is on my list to make something!
It would be interesting to know what you get.
You may have seen the chart attached; it seems the Germans could not decide which spring to use either!
(Obviously they used a different method to determine the "poundage".)