I want to use 8/2 cotton (that's what I have on hand) for the log cabin sample and understand that I need to add ends if I want it to come out close to the size the pattern as written would produce. I started to think it was as simple as doubling the number of threads in each section but then realized that if I thread it that way my dark/light and light darks will be off. Should I just thread 11,22,33,44 for these sections or can I just pull two threads through one heddle? Or will I be happier if I wait and order the 8/4 cotton?

You could follow the draft for the sample in 8/2 cotton - the size of your blocks will be smaller but you could increase them by instead of doing 5 alternating dark/light or light/dark - you could increase it from 10 ends to 20 ends until you get to the final red zinger. Then, when you weave it remember to increase your Tromp as Writ to include the changes you have made in the draft. Does this make sense?

I think I understand.
Instead of doubling ddll 5 times I would repeat DL 10 times.