If it's a D35, the tube diameter is 2.62". I've run longer ones from advanced auto on my old p.o.s. without issues. Just find what's closest and run with it. For what you have planned, I don't think it'll matter a whole lot.
Dave, take one off and run by Spring Rebuilders here downtown. They'll make them on the spot in a few minutes. Cool shop too. lots of good stuff to look at while you wait.