FLY, when I built my first vintage (Sig Ryan STA) kit back in 2012 I had the same concerns, in fact below is a direct quote from the first post of that build. I am now on my second vintage build and again all the wood is as good if not better than what you could have purchased today. I have not had any problems with glue-ups or wood splitting. I think it comes down to how the kit (wood) was stored, along with the temperature and humidity all those years. I would not hesitate to purchase your Proctor kit. I have two more vintage kits on the shelf for future builds!
"I've always admired the Sig Ryan STA plane, and recently I had the good fortune to buy one on E-bay for a tidy sum of $225.00. Now buying a NIB kit from 1971 makes one wonder if all that money that you are spending will just be for the plans and instruction booklet, after all, the wood in the kit is 41 years old! My kit arrived recently and I was eager to open it and check its contents.It was like a time capsule. I found the wood to be of the highest quality with no warping, twisting, splitting or hardening of any kind! I really couldn't believe it...."