noob needs help with mileage - only 10 MPG!

Once again guys, big thanks all around from this noob.

I'm motivated to learn, and its definitely worth trying the out simple/cheap/free fixes before going for and engine swap or rebuild.

One thing about that engine listed on Craigslist: it may not be any better than the engine I have now, right? The engine I have now seems to run well, and if I hadn't mentioned the low 10mpg to my mechanic he wouldn't have done the leak down test. In other words, that Craigslist engine may also get about 13mpg (which is what mine is getting after a carb overhaul and putting in an electronic ignition). Granted is hard to go wrong for $100 and doubtless I could use some of the parts on that engine.

If anyone reading this is in the greater Portland, Oregon area, please PM me. I probably can learn to do something like valve adjustment from a manual, but I'd really like to be able learn from someone who has done a lot of tinkering on their own slant six. If you're willing to help me learn how to do some of these things, I'm more than happy to supply as many pizzas and six packs as it takes.

Thanks again for all the informative replies.
I've learned a lot already from you guys and I greatly appreciate it.