: What constitutes "fairly close growth rings? I have what I bought below,
: don't know if it will attach successfully or not

While the growth rings on that board look pretty wide, I wouldn't sweat it. Grain orientation and straightness are much more important. The grain you have there is good (especially for a S/P/F 2x4), not great, but should be fine for your first paddle. It's unlikely that it will be your last and it will be a good learning experience, regardless. As you say, for two bucks, how far wrong can you go?

I've attached a shot of WRC boards with what I consider to be good grain. You'll notice that there is variation in angle and tightness of the grain, but they all make fine paddles. Not also that you can create vertical grain lumber by resawing flat grain lumber and laminating it, as was done in the bottom photo. That's two pieces of WRC laminated to a white pine "spine".